Decorators firstly agreed colours (4)
I believe the answer is:
dyes
'colours' is the definition.
(dyeing is a kind of colouring)
'decorators firstly agreed' is the wordplay.
'firstly' indicates taking the first letters.
'agreed' becomes 'yes' (I've seen this before).
The first letter of 'decorators' is 'd'.
'd'+'yes'='DYES'
(Other definitions for dyes that I've seen before include "Colouring materials" , "turns green, possibly" , "Colour" , "Substances for staining or colouring" , "Colours (fabric)" .)