If we're talking about the ashes that would remain after a human body was exposed to kiln temperatures (> 1000°C), then we are talking about the mineral content, i.e. the calcium from bones.
Bone is made of calcium phosphate, which is pretty inert for ceramics purposes; it won't melt or undergo chemical reactions in a kiln, so it can't be used to make ceramics on its own (if we were talking about seashells, made from calcium carbonate, it would be a different story).
However, bone ash can be incorporated into ceramics as a filler, in the same sense that raisins or nuts can be baked into any kind of cake or bread. But in the same way you can't make a cake from just raisins, you still need to mix bone ash with a significant proportion of clay. It is the silicates in the clay that melt and fuse to create a solid material when the pottery is fired.
At these temperatures, the ash will basically be white – the very small amounts of other inorganic elements like iron will not be enough to affect the color noticeably.