Allow coloring for black "line art only" stamps

Hello Inkarnate Team,

I would like to request a small feature addition for your map-making software: it would be incredibly useful if the black "line art only" stamps could also be colorable.

From a technical standpoint, this should only be a few minutes of development time. If it helps speed up the process, I am more than happy to provide the implementable code for this feature.

Thank you for your time and for the great software!

Best regards,

Benjamin