Who would prohibit the use of colour? (6)
I believe the answer is:
banner
'who would' is the definition.
I can't tell whether this definition defines the answer.
'prohibit the use of colour?' is the wordplay.
'prohibit' becomes 'ban' (banning is a kind of prohibiting).
'the use of colour?' becomes 'ner' (I can't explain this - if you can you should give a lot more credence to this answer).
'ban'+'ner'='BANNER'
Can you help me to learn more?
(Other definitions for banner that I've seen before include "Flag bearing a device" , "Streamer for advertising" , "Military standard" , "Ensign" , "Pennant" .)