Colors are supposed to follow the basic hyphenation rule: hyphenate adjectives before a noun, but not after.
(Read more about that rule in this post.)
But CMS and AP have some additional considerations that complicate things a bit. These are not clarified in the text, but are found when searching the Q&A sections or forum of each website. Here is what I deduced.
If 2+ colors are mentioned
(red orange, bluish green, black and white)
Hyphenate adjective before a noun.
Don’t hyphenate if it’s a noun itself.
Bluish green is my favorite color.
a shade of bluish green.
Don’t hyphenate adjective (or adverb) after a noun.
The water looks bluish green.
The water, bluish green and sparkling, beckoned me.
The water beckoned, bluish green and sparkling.
If it’s an adjective after a be verb (is am are was were be being been):
CMS still prefers two words.
The water is bluish green.
AP prefers a hyphen:
The water is bluish-green.
If it’s a color shade
(mint green, light blue, dark red)
CMS still follows the same rules as listed above.
mint-green paint, a shade of mint green, the paint is mint green.
AP prefers to leave it two words in all instances (unless it somehow causes confusion).
mint green paint, a shade of mint green, the paint is mint green.
Examples: 2+ colors
The water, blue green and sparkling, was a welcome sight.
Blue green is my favorite color. (noun)
CMS: The water is blue green.
AP: The water is blue-green.
a pinkish-red color
The paint is a shade of pinkish red.
CMS: The paint is pinkish red.
AP: The paint is pinkish-red.
The facts were right there in black and white.
CMS: The facts are black and white.
AP: The facts are black-and-white.
Examples: color shades
CMS: royal-blue jacket
AP: royal blue jacket
The jacket, royal blue with white trim, reminded me of my favorite baseball team.
The jacket was available in royal blue.
The jacket was royal blue.
CMS: dark-violet sky
AP: dark violet sky
The sky, dark violet and cloudy, was an ominous backdrop for the carnival.
The clouds threatened from a sea of dark violet.
The sky was dark violet.
These follow the regular hyphenation rule: hyphenate adjectives before a noun, but not after.
But with AP, hyphenate adjectives after a be verb.
a strawberry-colored shirt
Her shirt, strawberry colored and bright, made her stand out in the crowd.
CMS: The shirt was strawberry colored.
AP: The shirt was strawberry-colored.
Source: CMS 7.85: 1: colors. AP: hyphen