Translation:They can be red, orange, white, yellow and blue.
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Below, user Shizukani says...
'Some colors, like "orange" and "marron," are invariable, which means they don't change form according to the subject. See http://french.about.com/library/begin/bl_colors.htm for more information.'
The ones you list are the ones that are used most frequently but as it turns out there are lots of others (mostly adjectives derived from nouns, such as animals, flowers, fruits, gems, and metals). I found a list here: http://french.about.com/od/grammar/a/adjectives_inv.htm
I got this wrong by pluralizing the colours (as in they can be various shades of red, orange, ect) and got marked wrong because the colours here are plural to match 'elles'. How then would you say this to indicate they could be various shades instead of just one tone of red, yellow, etc?