A dirty shade? (4)
I believe the answer is:
blue
I believe this is a double definition.
'a dirty' is the first definition.
(blue can mean pornographic)
'shade?' is the second definition.
(I have seen 'A shade' mean 'blue' so perhaps 'shade' could also mean 'blue')
(Other definitions for blue that I've seen before include "Miserable - cerulean" , "as the sky, perhaps" , "Azure" , "Conservative > depressed" , "See 7 Across" .)