Colours are ADJECTIVES. When they describe a NOUN, they agree with its GENDER (whether it is masculine or feminine).
So, 'blue' is bleu, but becomes bleue with a feminine noun eg la chaise bleue (the blue chair). The same is true of other colours – for example, noir becomes noire etc.
bleu(e)
noir(e)
The masculine and feminine forms of some colours have different sounds:
vert -(m)