In colour, it is colossal (5)
I believe the answer is:
titan
'colossal' is the definition.
(thesaurus)
'in colour it' is the wordplay.
'in' means one lot of letters goes inside another.
'colour' becomes 'tan' (tanning is a kind of colouring**).
'tan' placed around 'it' is 'TITAN'.
'is' acts as a link.
(Other definitions for titan that I've seen before include "enormous thing" , "Giant of Greek mythology" , "One of the mythological Greek giants who ruled the world" , "A giant; Saturn moon" , "one of great importance" .)