Bonjour Aaron,

In French, 'marron' and 'orange' never change in any form, plural and/or feminine. There are part of a list of nouns that are used as adjectives of colour. As such, they don’t change form.

See link here: how-colour-descriptions-adjectives-change-according-to-gender-and-number

Attention: these following colours (taken from nouns) do now ‘behave’ as real adjectives ; therefore they do agree with the noun(s) they refer(s) to.

I hope this is helpful.

Bonne journée !