Colour between red and blue (6)
I believe the answer is:
violet
'colour between red and blue' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for violet that I've seen before include "plant" , "Shade of purple" , "Bluish-purple colour as of pretty flower" , "It nods on Oberon's bank" , "Mauve" .)