"Water is blue."
Translation:Akvo estas blua.
As posted above, you could use it to say the water is blue (la akvo bluas).
Bluas would be the verb form of blue. Maybe say something like "I blued the wall" (I [colored] the wall blue).
I hope this is correct, clear and helps someone.
Heavily edited: and thank you Sremac for a better answer.
I think that "I blued the wall." would be "Mi bluigis la muron.".
Esperanto is very flexible and it is allowed to make a verb from any adjective just by changing the suffix. Maybe such a verb does not exist in your language, but it can exist in some other . In my language it is usual to say that "la cxielo bluas, la herbo verdas kaj la pomo rugxas."
What with the inherent flexibility of Esperanto, I'm sure that colors can be nouns.
Blua - adj: blue
Bluo - n: the color blue
Blue - adv: "bluely"
Bluas - v: (It) is blue. / (It) blues.
Though those last two are probably not regular versions of the word, I believe that all are at least possible, especially in a poetic intention.
EDIT: I wrote "Akvo bluas." for this exercise, & it was indeed accepted.