It depends on the coal, the AFT, the appliance (stoker or hand-fired), reducing or oxidizing atmosphere, and how hot it's being fired.
The same coal fired in a stoker will produce crunchy clinkers but will produce nothing but baby powder ash in my hand-fired. In high excess air settings in my stoker the same coal will produce bright red iron clinker and red ash, but in minimal excess air settings will produce black iron clinker with grey ash.
Some high AFT low ash coals such as many found in eastern KY will produce a white ash with little to no shale or iron - it will look like wood ash in a hand-fired.
Basically, your question only has an answer if we know the type of coal and the appliance -even then it varries.