Yes, you can use colors as both comparatives and superlatives. -- The sky is bluer than the ocean. That is the bluest sky I have ever seen.

Mu's websites may describe some philosophical truth, but in language the forms black/blacker/blackest and white/whiter/whitest all exist. Detergent advertisements use all forms of white, and discussions of race use all forms of black. Jacob's teaching experience covers traditional primary colours such as red, yellow and green, whose comparatives and superlatives are commonly used. I'm not so sure about other colours, though: I can't imagine using them for cyan, magenta, orange, mauve etc., though I might try to use them in Scrabble.

grammatically and technically you can use any color/colour as a comparison or superlative, but in daily speech the common ones are white black, green and blue and red and yellow or pink as white as snow. as black as night. as green as grass. as blue as the sky. as red as a sunset/sunrise. as pink as a babies bottom "blacker than a coal miners face" "whiter than fresh snow" "bluer than a Mediterranean sky" "greener than lush grass" "redder than Rudolph the reindeers nose" "pinker than a babies bottom" for other colours people would not be able to know or imagine the comparison. So are not used.