I'm thinking browbands - is his face more white or more coloured? What 'skewbald' is it? Chestnut, bay or brown based?
No white, it will disappear into his white markings. Depending on how dark he is, you either want bright colours or darker neutrals. I have baby blue, chocolate and cream on my foal, she is solid coloured though. On my (again solid) bay I have navy, gold and cream or purple, white and silver, and both look fabulous. It depends on the horse. Greens and blues look fabulous on chestnuts and bays, traditional red/white/blue looks awesome on most colours (but not too much white in your case). For saddle blanket/bandages/halter you would want to pick a colour from the browband that contrasts both the markings and the base colour.
Is your cob a true tricolour or a bay and white pinto? On a bay and white pinto I would strongly suggest shades of blue or shades of green, perhaps with some red in there. Purple would also look lovely.