In french, adjectives change depending on the noun, they are used with. If the noun is feminine, the adjective has to be feminine. If the noun is masculin, the adjective is masculin. Also you have to pay attention to plural/singular business.
Masculine singular bleu
Feminine singular bleue
Masculine plural bleus
Feminine plural bleues
Note that rouge is same for masculin et feminine. Roux and rouge are somewhat different. Roux is used for describing the color of hair. Rouge is color red and can be used for describing anything.