Butterfly practically overflows with colour (9)
I believe the answer is:
brimstone
'butterfly' is the definition.
(I've seen this before)
'overflows with colour' is the wordplay.
'overflows with' becomes 'brims' (I can't explain this - if you can you should give a lot more credence to this answer).
'colour' becomes 'tone' (tone is a kind of colour).
'brims'+'tone'='BRIMSTONE'
'practically' acts as a link.
This may not be right. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for brimstone that I've seen before include "Biblical sulphur that rained with fire on evil cities" , "Yellow butterfly" , "Sulphur; butterfly" , "hellish punishment" , "It robs men (anag) - old word for sulphur" .)