My first Duo introduction to clair was clear, light and bright in that order.
Generally I try to stay from Duo's lower ranked definitions unless context seems to require it.
Duo's use of light for clair in the very short phrases they are using encourages students to think that clair as light is meant as light weight.
Sitesurf has indicated in another post that clair as light actually means light colored.
Four different dictionaries give uses such as light blue eyes, plain color, clear thinking, clear speech, a clear presentation. clear sound (which could be construed as bright). etc.
I'm reporting Duo's handling of clair as being deficient in several respects.
The compound adjective "light-coloured" is rightly hyphenated in attributive position ("A light-coloured dress") but there is disagreement about compound adjectives in predicative position ("The dress is light-coloured" or "The dress is light coloured").
It's not easy to reconcile the differing opinions on this point. My own preference is to omit the hyphens in predicative compounds where the meaning remains clear without them, though I am sure many examples can be found to show problems with that approach. One that pops into my mind is "multi-coloured" where multi cannot really stand alone, but there will be many others.
My point, really, is that "Her dress is light coloured" should not be marked WRONG. A minority view, perhaps, but mine own.