Punctuation – How to Correctly Use a Semicolon

This is a simple list of 3 elements, not a complex list of 2 elements whose second element contains two subelements.

Case 1

1. deep red with black with dark green or dark purple
2. medium shades of red with similar tones of green and orange;
3. etc.

Case 2

1. deep red with black with dark green or dark purple
2. medium shades of red with similar tones of
   1. green
   2. orange
   3. etc.

The semicolons make it clearer that case #1 applies than would the comma.

Contrast:

• People with dichromatic colour vision will usually find it difficult to distinguish between similar tones and shades: deep red with black with dark green or dark purple; medium shades of red with similar tones of green and orange; etc.

• People with dichromatic colour vision will usually find it difficult to distinguish between similar tones and shades: deep red with black with dark green or dark purple, medium shades of red with similar tones of green and orange, etc.

In the second version, it is far from immediately obvious that we are not talking tones of green, orange, and others. Using the semicolon makes this clear(er).

Personally, I don't think that either a semi-colon or an en-dash is appropriate in your examples. I would use a colon or an em-dash. (I also don't think that an en-dash v. and em-dash is an American v. British issue: they have different purposes (see http://www.chicagomanualofstyle.org/qanda/data/faq/topics/HyphensEnDashesEmDashes/faq0002.html).)

A semi-colon is part-way between a comma and a full-stop: it is not for juxtapositioning two clauses in the way you have -- that requires a colon or an em-dash!

This article http://www.thepunctuationguide.com/em-dash.html has a useful summary about em-dashes v. colons and other punctuation.