"Dark green is a nice color."
Translation:Tmavě zelená je hezká barva.
It's not a noun. "zelená" is always an adjective. However, it's the subject in this sentence, and subjects are usually nouns, that's why it may be confusing. It can help to imagine it says "Tmavě zelená (barva) je hezká barva." So, "zelená" in its feminine form, used by itself, is often a shorthand for "zelená barva" in certain contexts. The same is true about other colours of course.
It's just another way of saying the same thing.
tmavě zelená: "tmavě" is an adverb which modifies the adjective "zelená", i.e. "dark(ly) green", this is the most frequent way of expressing it in Czech
tmavá zelená: a green colour which is also dark, "tmavá" is an adjective modifying the full but unspoken expression "zelená barva" ("barva" is only implied here).
tmavozelená: another, albeit less frequent option, "dark-green"
"Tmavě zelená je hezká" is certainly a correct and valid sentence, but I'd say it's a translation of "Dark green is nice". Is that the same sentence?
"Tmavě zelená je hezká" probably refers to the "dark green color" as such, but I can imagine situation where it refers to something else: Let's say there are shirts of multiple colors laid out on a desk and somebody asks you "Která košile se ti líbí?" = "Which shirt do you like?" - you might answer "Tmavě zelená je hezká." = "The dark green one is nice." (It would more often be "ta tmavě zelená...", but not necessarily).