Voltaire exuding anger could become a sort of green (5)
I believe the answer is:
lovat
'a sort of green' is the definition.
(lovat is a shade of green)
I can't explain the remainder of the clue.
(Other definitions for lovat that I've seen before include "(Of woollen cloth) muted green-blue colour" , "Green used in tweeds, etc" , "sort of green" , "Green colour, as of tweed" , "Green cloth" .)