Immense without key colour (3)
I believe the answer is:
hue
'colour' is the definition.
(I know that hue is a more specific form of the action colour)
'immense without key' is the wordplay.
'immense' becomes 'huge' (synonyms).
'without' suggests deleting specific letters.
'key' becomes 'g' (musical key).
'huge' with 'g' taken out is 'HUE'.
(Other definitions for hue that I've seen before include "appearance" , "Cast" , "Tone, shade" , "Colour or tint" , "Tint, shade" .)