Translation:The shirt is available in white or blue.
The only agreeing adjective is "disponible", and "en blanc ou en bleu" is an adverbial phrase.
"La chemise est blanche ou bleue" vs "la chemise est disponible en blanc ou en bleu"
Probably not all; it will depend on what they consist of. In this phrase "blanc" and "bleu" refer to the color white = le blanc, and the color blue = le bleu. Color nouns are indeed in masculine.
It does not matter what we call it, what matters is that you now know that colors can be nouns, and in masculine.
Well, I was thinking blue and white were not nominatives but adjectives describing the elided [la couleur]. So how would the F go if the E was "available in the color blue or the color white?" I guess we have to live with the reality that "Le bleu est une couleur."