Blue is as even but not as much (4)
I believe the answer is:
less
'not as much' is the definition.
(similar in meaning)
'blue is as even' is the wordplay.
'even' means one should take alternating letters (letters in even-numbered positions are used).
'blue'+'is'+'as'='blueisas'
The alternate letters of 'blueisas' are 'LESS'.
'but' is the link.
(Other definitions for less that I've seen before include "Comparatively little" , "A smaller amount" , "more, as they say" , "Wanting" , "minor" .)