The general tendency (which is not the same as "an accepted approach") that I've observed in publishing over the past 30 years is toward less hyphenation of stacked adjectives, especially when one of the adjectives essentially modifies another.
If two words possess equal force as modifiers (as is the case with blue-gray in the phrase "blue-gray gnatcatcher"), it makes sense to reflect the balance between the two words by hyphenating them when they appear in combination. But if one adjective dominates the other (as is arguably the case with, say, "bluish gray clouds," where the main color of the cloud is gray but that color is tinged with blue), I don't see what you gain by including the hyphen between bluish and gray.
You can apply a similar analysis to "light brown hair" and "dark brown hair": The dominant adjective in both cases is brown, with dark or light modifying that color. Under the circumstances, it makes perfect (and consistent) sense to render the two phrases without hyphens.
As for your concern that readers might misread "light brown hair" as meaning hair that is both lightweight and brown, the absence of a comma after light signals to readers that you aren't using light in the sense of "lightweight"; after all, lightweight would modify hair, not brown, so you'd want to isolate it from brown as clearly as possible to indicate that it referred directly to hair.
None of this is graven in stone, of course. You can find lots of published instances where stacked adjectives are variously hyphenated, unhyphenated, commaed, and even slashed to serve the taste of the author, editor, or publisher. And as Robusto notes in a comment beneath your question, the whole issue is ultimately a matter of style anyway—and style rules tend to be will-o'-the-wisps.
Still, if you're looking for an approach that aims to produce results that are generally consistent and fairly rational, you could do worse than to base your hyphenation of stacked adjectives on whether the adjectives represent coequal merged qualities (as in "blue-gray gnatcatcher") or whether one adjective simply modifies the other (as in "bluish gray clouds").
I mean is it preferable to say:
red and white flowers Or:
white and red flowers
Or is there really no grammatical rule to obey?
No, there is no rule, you can use any order you like:
"There were white, red, purple and pale-blue flowers on the table" is correct with any permutations of the colours
Best Answer
Let's say the noun Joe wants to describe is the paper he just wrote. It has more pages than the average, so one adjective is long. We want to modify the way "long" describes the "paper", so we look for some words to make the modification.
Do we want an adjective? No, because "long" is not a noun, though it can be made a noun when you change it to "length". Rather, it is doing something to the word "paper" by describing it, so if we want to modify the description, we need to add an adverb.
For example, if we want to clarify that it is not necessarily among all papers that Joe's is regarded long, but only among this topic, then we might say "Joe wrote a relatively long paper".