Use a lot of each colour filling in box (3,4)
I believe the answer is:
eat into
'use a lot of' is the definition.
'each colour filling in box' is the wordplay.
'each' becomes 'ea'.
'colour' becomes 'tint' (tint is a kind of colour).
'filling in' means to look at the middle letters (the content or filling of the word).
The middle of 'box' is 'o'.
'ea'+'tint'+'o'='EAT INTO'
(Other definitions for eat into that I've seen before include "Etch" , "run down" , "Consume (resources)" , "start wearing out" , "Corrode (something)" .)