Scarlet and crimson (4)
I believe the answer is:
reds
'scarlet and crimson' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for reds that I've seen before include "Colours for communists" , "First snooker balls to be potted" , "These are not right" , "Colourful communists" , "Scarlet communists" .)