Workflow was all over the place with this one, but the result had me very excited. From there I dropped it into Aesprite to pixel-push the edges in order to get it to x tile. I made sure to crop it to a power of 2 and then I took it into Pixatool and did things like: index it to the doom pallet, colour/lighting/sharpness and general destructive effects, and crushed it down to 256x256. This was an initial test, so I would frame and light the existing model, pose the camera and render the image at a high resolution. : Oh, oops! I forgot to post the GIMP file.

The issue with Doom's regular grout is that it is too grid-like, real grout tends to be irregular and thick, because it is what joins the bricks together it's not very easy to explain, but hopefully it'll be easier with this. ² More info on the Doom palette in GIMP here. ¹ Downscale interpolation never blurs, and it'll smooth many potentially distracting straggler pixels that way too From there you can export to PNG, or select all (Ctrl+A), copy (Ctrl+C), and then in Doomworld or Imgur etc, paste as an image buffer (Ctrl+V). Then, merge all layers (Ctrl+M), downscale to 256x256 setting the interpolation to Cubic¹, and set it to be the Doom palette² using Image -> Mode -> Indexed. Grab this file, extract it, and open it in GIMP, then use the bucket fill tool to change the colour of the entire 'Colour' layer (within the 'Highlights' layer group). To change the highlight colour is simple and easy if you already have GIMP installed. I even have a variant with strong yellow for big daddy Ribbiks~ I also sprinkled some slight highlights, one in bland white (for colored sectors), another in blue, and a few more with other such colours.

I used the good old nearest-neighbors trick to add lighting smoothly. No smoothing or shading, just wanted to see how it looked. Took your base and added the standard grout from BIGBRIK2.

Pixatool export