Blush (6)
(Other definitions for colour that I've seen before include "Hue, tint" , "Blue or green, say" , "Hue, shade" , "black?" , "Quality that results from light reflection" .)
'blush' is the definition.
(I know that blush can be written as redden)
This is all the clue.
(Other definitions for redden that I've seen before include "Group of states under one head" , "Flush, become embarrassed" , "Evaluate the nature of (something)" , "Give visible sign of embarrassment" , "Reveal embarrassment in colourful way" .)