Rave about colour that's declared (7)
I believe the answer is:
averred
'declared' is the definition.
(averring is a kind of declaring)
'rave about colour' is the wordplay.
'about' is an anagram indicator.
'colour' becomes 'red' (red is a kind of colour).
'rave' anagrammed gives 'aver'.
'aver'+'red'='AVERRED'
'that's' acts as a link.
(Other definitions for averred that I've seen before include "Stated, asserted" , "Alleged, asserted" , "confidently stated" , "Claimed" , "Maintained" .)