What causes the color of ash from wood charcoal?

Wikipedia says,

"The composition of wood ash is influenced by the type of wood that has been burned. Also the conditions of the combustion affect the composition and amount of the residue ash.

Wood ash contains calcium carbonate as its major component, representing 25 or even 45 percent. Less than 10 percent is potash, and less than 1 percent phosphate; there are trace elements of iron, manganese, zinc, copper and some heavy metals. However these numbers vary as combustion temperature is an important variable in determining wood ash composition."

I would assume these factors and others like density could account for any color differences in the ash. Potash is brown so if there was a large amount of potash in your ashes they might seem brown. When the same wood turns up different-colored ash, it sounds like it may be because of differences in burning temperature. I don't know if you could easily change the color by adding anything to the charcoal. It sounds like a lot of the compounds that could give the ashes any color besides gray are volatile and would mostly evaporate out as the wood burned leaving very little left. Page nine of the FPL article has a chart of six woods and the composition of their ashes if you are curious.