By Mark Nichol
Use of hyphens and commas in phrases that include names of colors is the cause of some confusion among writers. Here’s a discussion of when to insert or omit these punctuation marks when referring to colors.
As with most other phrasal adjectives, pairs of words that together describe the color of an object should be hyphenated: A suit that is dark blue (referring to degree of saturation) is a dark-blue suit, and a suit that is blue gray (identifying a combination of colors) is a blue-gray suit. (Note, too, that a modifying phrase referring to color, like most phrasal adjectives, is not hyphenated when it follows rather than precedes the noun it modifies.)
The previous rule applies not only to combinations of colors but to degrees (“greenish-blue dress,” “a dress that is greenish blue”) or comparisons (“snow-white fabric,” “fabric that is snow white”) of color.
Remember, too, that light has two distinct meanings: A light green package is a green package that doesn’t weigh much; a light-green package is a package that is a light shade of green. When the adjective or phrasal adjective follows the noun or noun phrase, and no hyphenation occurs, the distinction is still clear: The first description is of a green package that is light, and the second is of a package that is light green.
When the name of a color is one of two or more adjectives preceding a noun, whether and which of the adjectives are separated by commas depends on whether they are coordinate adjectives or not — whether they each modify the noun, rather than one modifying a phrase consisting of an adjective and a noun.
For example, in “a tall, green pole,” a comma separates the two adjectives because they are coordinate, or equivalent. See More . . .