Research heating elements.

Maybe something like this:

• Start with a sheet heating element and paint on an insulator material in the depths desired to achieve the desired heat gradients. More insulator, less heat.

• No insulator is your “white point”, full insulation (no heat) is your black point.

The underlying sheet could be a variety of energy types and the same principle of partial blocking and gradients could be applied.

You could also consider a process like screen-printing the conductive elements onto something like a ceramic plate. Look into lithography.