Crimson and scarlet (4)
I believe the answer is:
'crimson and scarlet' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for reds that I've seen before include "Scarlet and crimson, say" , "Scarlet communists" , "These are not right" , "Colourful communists" , "Colours for communists" .)