First of all, if you’re buying a new pot because you need something larger, only go up one size.  While you’re going to offer your tree a little more root space, it’s not like all of a sudden you’re going to encourage it to grow like crazy. It is after all – a bonsai.   If you’re growing a fruiting bonsai, you’ll need a pot that’s on the deeper side since the swelling fruit will need to have more water held in the soil.  Cascading and semi- cascading trees will need a taller pot that is a better match for stems that trail down the side.  Shallow, light or pale colored pots – oval and rectangular are well suited to plantings of deciduous trees while a heavier drum shaped or round bonsai pot is great for a single more rugged tree.  I like conifers in unglazed pots and while it is true that some of the flowering bonsai can be displayed most advantageously in a colored, glazed pot, it’s important that it doesn’t dominate the tree.