Red to unglazed brown to complement the green foliage. Or grey to complement the bark. Or beige to yellow to complement the autumn foliage. Or a mixture in glazes.
Length just within the foliage at either side, depth just over the thickness of the trunk.
Davidlpf good illustration to follow. If you're looking for a portrayal of a tree in a meadow that changes the length but I'd stick to my suggestions for colour. The last pot is a good shape for the growth pattern of your tree, strong, feminine. But I'd prefer rectangle.