Well like door pieces, yes they can be changed or painted, as for the seats I wouldn't paint them, but maybe look at changing just the leather out for another set of one's that are just black, you know what I'm saying? Just take the leather off not the foam and whole seat.
As for the dash and other trim panels yes there is ways to paint them, check out our how-to's and it will tell you the whole sanding and priming before paint process.
EDIT: here we go, this is how to change the color of the door material. HOW TO-Make custom inserts for door panels
And with this link here, you can find the rest of the ways to change the inside of your car.> * * * * C o m p l e t e ' H o w T o ' A r c h i v e * * * *