What did Adam Smith mean by “wretched natives of singular colour”

To give a sense of how the phrase "singular colour" was understood, in the eighteenth century before and after publication of Adam Smith's The Wealth of Nations in 1776, I offer the following instances, located though a Google Books search.

Instances from before 1776

From Robert Drury, Madagascar: Or, Robert Drury's Journal, During Fifteen Years Captivity on That Island (1729):

They [the natives of a remote part of Madagascar] never trouble themselves, who is Lord of any particular Place, or King of the whole. Deann Murnanzack's Cow-keeper, who was my Governor, had in the former Part of his Time liv'd on this Manner, and was, therefore, acquainted with some of their private Settlements ; he conceiv'd that I, being a Man of a singular Colour, might be impos'd on them for a Prince of Murnanzack's family, they being very ignorant ; an yet not more so than some vulgar People in Europe, who imagine that their Princes are something more Excellent, or Extraordinary in their Make, or Shape, from the rest of Mankind.

The author was an Englishman and presumably would have stood out from the native people of Madagascar in eye color, hair color, and skin color.

The next several entries are from a single author, John Hill, who invariably uses "singular colour" as part of a longer phrase: "a very singular colour." From John Hill, A History of Fossils (1748):

5. Saburra tenuior, spledida, ferruginea. Fine, shining, ferrugineous Gritt.

This is a very bright and glittering mass, and of a very singular colour. It is considerably fine, extremely heavy, and of a colour resembling that of rusty iron, but much darker, and without the yellowness that has, and spangled all over with very bright glittering particles, which might easily be mistaken for flakes of Talcs, but are in reality no other than pure and colourless crystalline fragments ; its particles are very equal in size, tho' so different in colour.

From John Hill, An History of Animals (1752):

This [the Spanish locust] is about the length of the former species [the mole cricket], but it is not nearly equal to it in thickness : it is of a very singular colour, a deep brown, with an admixture of a whitish grey ; and there are a great number of little white dots at the origin of the wings : the exterior wing are beautifully spotted with little dots of black ; the interior ones are reticulated : ...

...

The head [of the amelis] is of a pale chestnut colour on the forepart, but behind it is of a much deeper brown, approaching to that which our painters call an umber colour : at the sides there are two black spots, formed, as it were, of a continuation of the bristly covering of the nostrils, and in these stand the eyes : the neck is short ; it is black before and behind, but of a strong, though not not very deep brown, at the sides, and, as it approaches the head, it becomes whitish : the shoulders and back are of a deep reddish-brown, a very singular colour, and very elegant ; the brown is bright, though strong ; and the reddish that i blended with it is not of a scarlet or crimson kind, but of the pale blossom hue, or flesh coloured tint : the breast and belly are of two different colours ; the breast a ferruginous brown, deep and shining ; and the belly a pale, but not bright, grey, with no tinge of either brown or red in it ; there is a shade of chestnut colour visible in certain lights, though not in all ; and this is most distinguishable about the middle of the back ; the feathers about the rump are yellowish.

...

This [the garagney] is about the bigness of the common teal, and in many particulars it greatly resembles it : ...the top of the head is black, and there is a white line on each side, running from the eyes quite down to the neck, and reaching to it's middle : the throat is black ; the breast is variegated in an undulatory manner, with black and grey ; the back is of a very singular colour, deep brown, with a tinge of purple thrown over it : the feathers which cover the thighs are also variegated with black and white.

...

The back of the neck [of the summer teal], and the whole upper surface of the body, the back wings, sides, and rump, are of a very singular colour, a reddish-brown ; the red seems of a deep hue, and is so perfectly blended with the ground colour, that it scarce any where discovers itself in any light ; and the result of the perfect union is a peculiar colour, approaching to what we express by the term copper colour ; it is unlike any thing that we see elsewhere in the colouring of the duck-kind, and always claims attention : the wings are moderately long ; the tail is short ; the breast and belly are of a pale colour, and the legs, which are short, and not very robust, are of the same greyish-blue with the beak ; but the membrane which connects the toes, and forms the web of the foot, is black.

...

This [the regulus without a crest] is as small as the preceding species [the crested regulus], and is a very elegant little bird : the head is small, and the eyes of a bright hazel ; the beak is very slender and sharp, and of a bright brown : the head, neck, and back are of a very singular colour, which seems composed of grey, green, and brown ; upon the whole it forms a very singular kind of olive : the sides of the head are ornamented with an oblong yellow line, which runs from the eyes to the hinder part of the head : the throat is of a very pale yellowish, approaching to lemon colour ; the breast and belly are of the same colour, only yellower : the long feathers of the wings, and those of the tail, are of a dusky brown, but they are somewhat brown at the edges.

...

The fur upon the whole animal [the lynx] is very long and deep, but is not so thick as in many of the preceding species [lion, tiger, leopard, and cat-a-mountain] ; it is of a very singular colour, a kind of a very pale reddish brown : there is an admixture of white in it in some parts, and the whole body and legs are spotted with black : the spots are of an irregular figure, and stand at distances, and are not large.

From John Hill, Eden: Or, a Compleat Body of Gardening (September 1757):

The Leaves [of the ciliate leav'd rhododendron] are cluster'd on the upper Parts of the Branches, and are very singular and elegant. They are small, of a lanceolate, but nearly oval Form, and of a very singular Colour ; a brownish red or ferrugineous Hue, with very little green ; shining and glossy on the Surface ; and at the Edges surrounded with regular, stiff, dark Hairs, resembling Eyelashes.

From The Adventurer, Number 80 (August 11, 1753):

Greatness, novelty, and beauty, are usually and justly reckoned the three principal sources of the pleasures that strike the imagination. If the ILIAD be allowed to abound in objects that may be referred to the first species, yet the ODYSSEY may boast a greater number of images that are beautiful and uncommon. The vast variety of scenes perpetually shifting before us, the train of unexpected events, and the many sudden turns of fortune in this diversified poem, must more deeply engage the reader, and keep his attention more alive and active, than the martial uniformity of the ILIAD. The continual glare of a singular colour that unchangeably predominates throughout a whole piece, is apt to dazzle and disgust the eye of the beholder.

This instance is interesting in that "singular" seems to carry a sense of monotonality that is absent (or less evident) in the other examples gathered here.

An instance from after 1776

From Thomas Pennant, History of Quadrupeds (1781):

The antients did not neglect experiments whether they could not improve the breed. Columella says, that his uncle, M. Columella, a man of strong sense, and an excellent farmer, procured some wild rams, which had been brought among other cattle to Cales from Africa, by way of tribute, which were of a very singular color. These he turned to his common sheep. The first produce was lambs with a rough coat, but of the same color with the rams. These again produced, from the Tarentine ewes, lambs with finer fleeces ; and in the third generation, the fleeces were as fine as those of the ewes, but the colour the same with that of the father and grandfather. This breed was the same which the old Romans called umbri or spurious.

Conclusions

In the examples cited above, the phrase "singular colour" (or "very singular colour") seems most often to indicate a meaning along the lines of "striking, unusual, or remarkable colour." The instance from The Adventurer differs from the others in using the phrase figuratively to suggest a narrative that has a strong element of sameness at the same time that it may also possess vividness and (as the author puts it) "glare."

Samuel Johnson, A Dictionary of the English Language (1755) lists five definitions of singular:

SINGULAR. adj. {singulier, Fr. singularis, Latin.} 1. Single; not complex; not compound. ... 2. {In grammar.} Expressing only one; not plural. ... 3. Particular; unexampled. ... 4. Having something not common to others. It is commonly used in the sense of disapprobation, whether applied to persons or things. ... 5. Alone; that of which there is but one. ...

I think it is very likely that Smith is referring to either definition 3 or definition 4 (in a nonpejorative sense) of singular in the phrase "singular colour and appearance." I am also fairly sure that the "colour" he has in mind is the natives' skin (and possibly eye and hair) color.