"Their bags are purple."
Translation:As bolsas delas são roxas.
Many colour words do not vary with gender or number. Usually they are the ones that describe the colour of a particular substance or object. A good example is orange which in full is "cor de laranja" (the colour of an orange) and you use: "sapato laranja", "sapatos laranja", "luva laranja" and "luvas laranja".
It turns out "púrpura" is one of these words and "cor de púrpura" is the colour of a secretion produced by a type of snail (strange as it may seem).
There are several problems with that: Firstly, "bolos" means cakes not bags (perhaps you meant "bolsos", but that's pockets not bags); secondly, "vocês" is a subject pronoun meaning you (plural) not a possessive pronoun; and thirdly, "bolos" is masculine so should be paired with "roxos" not "roxas".
Your sentence would make more sense as "As bolsas de vocês são roxas" (Your bags are purple) but it still doesn't translate Duolingo's English version.
I'm not really sure what you are asking so apologies if this does not help.
When a colour is used as an adjective it can have up to four forms to match the gender and number of the noun it describes: "saco roxo, bolsa roxa, sacos roxos, bolsas roxas".
Note, some colours only vary with number: "saco/bolsa azul, sacos/bolsas azuis" and some don't vary at all: "saco(s)/bolsa(s) laranja".