Shrub inapt? Dreadfully — this might add colour (10)
I believe the answer is:
paintbrush
'this might add colour' is the definition.
(a paintbrush adds colour to something)
'shrub inapt? dreadfully' is the wordplay.
'dreadfully' indicates an anagram (a dreadful spelling).
'shrub'+'inapt'='shrubinapt'
'shrubinapt' with letters rearranged gives 'PAINTBRUSH'.
(Other definitions for paintbrush that I've seen before include "Artist's tool" , "Artist's applier" , "applicator such as this" .)