Brownish yellow in browns and yellows! (5)
I believe the answer is:
sandy
'brownish yellow' is the definition.
(sand-coloured)
'in browns and yellows' is the wordplay.
'in' indicates a hidden word.
'SANDY' is hidden within 'browns and yellows'.
(Other definitions for sandy that I've seen before include "Alexander (abbrev.)" , "Like a beach perhaps" , "- - Lyle (golf)" , "gritty" , "Hair colour" .)