Colour of muddy earth (3)
I believe the answer is:
dye
'colour' is the definition.
(I know that dye is a more specific form of the action colour)
'of muddy earth' is the wordplay.
'of' indicates the answer is hidden within the clue.
'DYE' is hidden within 'muddy earth'.
(Other definitions for dye that I've seen before include "Colouring liquid" , "Tint" , "Pigment" , "Colour (fabrics)" , "Colour or stain something" .)