Colourful but dirty (4)
I believe the answer is:
blue
This might be a double definition.
'colourful' is the first definition.
Both the definition and answer are adjectives. Perhaps there's a link between them I don't understand?
'dirty' is the second definition.
('blue' can be similar in meaning to 'dirty')
'but' acts as a link.
(Other definitions for blue that I've seen before include "Sky-coloured" , "Such humour < makes conservative" , "Red-headed Australian" , "Miserable; waste (money)" , "Colour - smutty" .)