Although the plural agreement indeed applies here, it would be worth mentioning that compound colour adjectives such as « vert émeraude » and « violet foncé » are invariable, no matter the grammatical number of the noun:

les courbes rouges et bleues

{vs}: les courbes rouges et vert émeraude

On another note: This is not the case with your example where « rouge » and « bleu » are separate colours, but if you are dealing with a single colour made up of multiple words connected by « et », such as « poivre et sel », then the adjective stays singular, even if the noun is in the plural:

les courbes rouges et bleues

{vs}: des cheveux poivre et sel

The plural looks more natural and is what most native French would use:

Les courbes rouges et bleues.

However, the singular is possible and even mandatory in that particular case given the fact it complies with the logic: there is only one red curve and only one blue curve.

Les courbes rouge et bleue.

like we would say, if talking about two and only two couples:

Les couples royal et présidentiel.

See http://bdl.oqlf.gouv.qc.ca/bdl/gabarit_bdl.asp?id=1744

Note also that there are several invariable color adjectives built from nouns, so whatever the number of orange or brown curves, you must always write:

Les courbes orange et marron.

and there are exceptions to these exceptions…:

Les courbes roses et pourpres.

For what you want to describe, you have to write:

Les courbes rouge et bleue.

As there is one red curve and one blue curve. In this simple case color adjectives behave like normal adjectives so you have to make them agree with the feminine singular.

The other possibilities mean different things:

Les courbes rouges et bleues.

means you are talking about several red curves and several blue ones, but basically the same grammar rules apply.

You could also see:

Les courbes rouge et bleu.

which means that each curve is both red and blue. Here “rouge et bleu” is considered as one complex color and thus there is no agreement.

You can find a source for example in this newspaper article, or if you want something that makes more authority Le français correct by Maurice Grevisse.