They are used with the singular neuter article "le" before a singular noun:
* Le rouge (the red)
* Le bleu (the blue)
And with the plural neuter article "les" before a plural noun:
* Les rouges (the reds)
* Les bleus (the blues)
It's important to note that some colors, such as rose (pink) and orange (orange) can be used as masculine nouns when referring to the color itself, but this is not the case when referring to the color of something else.
For example:
* Le rose est une couleur douce (Pink is a soft color) - Here "rose" is a noun
* Le rose de la robe est magnifique (The pink of the dress is beautiful) - Here "rose" is an adjective
Let me know if you have any more questions about French grammar!