I know with my piebald he looses the black and goes a mottled brown colour and he looses his shine a bit. I still clip him as I think its worse for them to get too hot but I also only do a bit of showing at the odd local show for experience as he is 4, don't do it "properly"! I suppose its like clipping in winter once the coat has gone they loose their colour for example chestnuts going dark, bays a brown/grey colour and greys look less dappled? I am working on assumptions here though!
I have to say though after 3 weeks the coat looks black and white again on him and was exactly the same for my old tri-coloured cob who I clipped all year round! So I just make sure I clip 3-4 weeks prior to any shows.