You must be occupying doctor as one changing colour (4)

I believe the answer is:

dyer

(Other definitions for dyer that I've seen before include "Colourer (of clothes, perhaps)" , "Person colouring cloth" , "Fabric stainer" , "One colouring e.g. fabric" , "Colourer of fabrics" .)

I've seen this clue in The Times.

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