Upscale a grayscale map and force palette to 3 colors
I have a low resolution map from a pdf and I want to use GIMP to standardize the grid at 50 px squares(no problem here) and turn the dark colors to black lighter grays to white and middle of the road grays to gray. I have made the 3 color palette, but it is not included in the selection for Image-->Mode-->Indexed Mode. So, how do I accomplish this?
The end goal is to then change the black to a rock like texture and the white to some dirt/cave floor texture to make it stand out in a VTT.