By and large, pot size and color are your choice.
Bonsai as an art combines several things. We try to make the tree grow robustly in a pot that matches the size of the tree, and there are rules which I have never been able to remember about size, shape, color, etc. We think, usually, that the tree should be in a pot with relatively shallow soil, which usually means 1 inch to 3 inches. Or, 2.5 cm to 3 cm or so.
But, reality means that the soil has to be deep enough to hold water during the summer days. Reality also means that any new tree I get will go into one of the existing dozen or so pots in the garage. I will probably try to match style, etc., but it may not be very close to the ideal.
My first bonsai book, about 1969, said pots should be unglazed brown, and flowering trees might be in dark glazed, probably blue. Today most people have their trees in glazed colored pottery. Times and styles change. Look at a lot of pictures on line, go to the local store, see what they have, and make yourself happy with whatever you pick.
