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.

Use 'blanche' after any feminine word like 'la couleur' (= colour): la couleur blanche. (it is used here as an adjective). However, 'le blanc' is used when you want to refer to 'white' in general. (it is a noun here) ;)

You should use 'blanche' when the noum is female: La maison est blanche. La voiture est blanche. You should use blanc when the noum is male: Le crayon est blanc.