It's used when describing weight or degree of simplicity, but not for colours in the way it is done in English.
The only way it can be used to describe colours, is in combinations like "lett rødlig" or "lett gulaktig", which would translate as "lightly redish" and "lightly yellowish" in English. It's not common, but it's the exception to the rule.
And the dictionary is always 100 % correct?
I had to google the rest of the colours, and with the exception of grey, the usage is about 50/50. Shouldn't spelling reflect what's common rather than what one dictionary says?
"Grønn er en farge" (1170) "Grønt er en farge" ( 1220)
"Blå er en farge" (1910) "Blått er en farge" (1610)
"Rød er en farge" (1200) "Rødt er en farge" (1740)
"hvit er en farge" (1570) "hvitt er en farge" (1650)
"grå er en farge" (631) "grått er en farge" (1390)
"brun er en farge" (443) "brunt er en farge" (501)
Unrelated: "Svart" is misspelled on the Tips and notes section, it's currently spelled "svartt".
I'm not a linguist, but shouldn't it be possible to construct sentences such as these, without them being ungrammatical?
[Adjective] is [noun]?
That is to say:
"Vakker er et ord" "Blå er et ord" "Blå er en farge"
(Plus, as fveldig says, it sounds completely natural to me to say "Gul er en lys farge".)
I answered "Yellow is a bright color" I double-checked this answer and was told it was correct, yet here, Duolingo is marking it as wrong. Please - either explain in the hints why "lys" does not mean "bright", and what the word for "bright" should be in Norwegian; or fix this mistake.