Colours are adjectifs. Adjectifs,  in the French language, follow the noun it is attached to ; both in number and in gender.

As colours are adjectifs, they will generally, agree with the gender and the number of the noun it is attached to. 

And the position of the colours, unlike in English,  is after the noun

Examples : 

	un papillion bleu		deux papillions bleus
	une lettre bleue		deux lettres bleues