Unbelievably bad bile, out in colour (4,4)
I believe the answer is:
navy blue
This could be a double definition.
'unbelievably bad bile out' is the first definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'colour' is the second definition.
(navy blue is a kind of colour)
'in' is the link.
(Other definitions for navy blue that I've seen before include "Senior Service shade" , "Armed service shade?" , "RN uniform colour" , "Colour of RN dress" , "Shade - venal buy (anag)" .)