just went to see a cossie today and it was a bit lumpy on idle , it was going up and down but it wouldnt cut out

it sounded really nice and was not tapping. its stage 2 light greens chip -31 scorpion exh etc 300bhp no dumpvalve only the standard one.
now the bloke said this was ok its just because of the light greens if i was to fit dark greens on, it would idle better, is this TRUE
just need a bit of advice peeps !
it was a 4x4 sapphire cosworth.
any advice appreciated
cheeRS