Blush, colour up (6)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
redden
(Other definitions for redden that I've seen before include "Flush, become embarrassed" , "Reveal embarrassment in colourful way" , "Evaluate the nature of (something)" , "Blush like the dawn sky" , "Group of states under one head" .)