"His color is apple green."
Translation:Sa couleur est vert pomme.
That's the way they do it in French (noun before adjective). In this case, it's a bit confusing, of course, because "vert" is usually an adjective, and "pomme" is, naturally, a noun. But if you replace the pesky adjectivesque "pomme" in this phrase with a different qualifier, like "foncé", it follows the normal noun-adjective pattern: "vert foncé".
Other colours are the same: "bleu ciel", "rouge cerise", "jaune beurre", etc.
The examples given aren't "blue sky, red cherry, yellow butter" (as in, a sky that is blue). They're "sky blue, cherry red, and butter yellow" (as in, a specific shade of the color blue or red or whatever).
Our sentence isn't talking about an apple that is green (pomme vert), but rather about the color apple green (vert pomme).
The color green (vert) as a noun is masculine... it doesn't matter about the subject. I believe if we were actually talking about a "green apple" it would be "pomme verte" (note the feminine version because "pomme" is feminine), but since we are talking about "apple green", the color, it's "vert pomme".
if "his color is green", then "sa couleur est verte" isn't the point here "la couleur" . so: "his color is apple green" -> "sa couleur est verte pomme" ? If not, does it mean everytime a color described with something (apple, sky, etc), we forget about the subject (la couleur)?
Hm... that's actually a good question. Assuming that "vert" is the noun here, one would think that it would have a noun-marker, such as le, la, l', un, or une. Perhaps it's describing the color, rather than stating it? In which case the phrase "vert pomme" as a whole acts as an adjective.
To exemplify this, you could say, "Elle a une couleur vert pomme", or "It has an apple-green color".
Quick edit: Just found this about.com article which may help. http://french.about.com/od/grammar/a/adjectives_inv.htm