I decided to give you here after the true grammatical rule about plural (and feminine) of adjectives expressing colors, because I could read here a lot of wrong assessments.
Adjectives expressing a color are usually taking the gender and the number of the noun they qualify, exception is made for those adjectives that are describing their color through the precise name of a matter, object, wood, fruit, flower etc that express better the true color to describe. In that case, the adjective means "of the color of an...) and is thus invariable.
You will say:
des cerises rouges,(red cherries)
des fleurs blanches (white flowers)
but you'll say
les coccinelles orange (of the color of an orange)
des marbres émeraude (marbles of the color of the emerald)
des meubles acajou (furniture of the color of the acajou (a tree))
des robes indigo (skirts of the color of the indigo (a matter extract from a plant)
but again there are exceptions as for colors expressed by flowers:
we will say
des roses roses, rouges, blanches, etc (pink, red, white roses) because here it means the color pink and not the flower itself as it can have different colors. The name in itself is not describing the color.
in the same way we will say
des reflets pourpres (purple reflects) eventhough "la pourpre" was a tincture extracted from a shell
I know French is awful to comprehend eventhough it's my language I have also to go back sometimes to my old grammar to remind me the exceptions in the exceptions.
Good luck to you and keep on!.