"Dark green is a nice color."
Translation:Tmavě zelená je hezká barva.
I don't understand the ending of tmavě. Which case / declination does it follow?
yes, zelená is a noun here. To be exact it is an adjective turned into a noun (zpodstatenělé přídavné jméno). Maybe that is why an adverb preceeds it where an adjective would if it was a regular noun.... I just made this theory up, so do not write it down with ink.