Here is my take on it. Much the same as PixLab.
Make your grid in a new transparent layer. Duplicate the layer. Then back down to the first grid and make that active. That will keep the grid nice and sharp.
Into the bucket fill tool and choose Fill by line art detection Still in tool options set Line Art Detection source to Active layer.
Then start fill in the grid. You can change colours using the FG / BG swatch or the colors dock as PixLab shows or bring up the palette edit dock and pick from that. Up to you.
50 second animation. https://i.imgur.com/xHcPyvj.mp4
Make your grid in a new transparent layer. Duplicate the layer. Then back down to the first grid and make that active. That will keep the grid nice and sharp.
Into the bucket fill tool and choose Fill by line art detection Still in tool options set Line Art Detection source to Active layer.
Then start fill in the grid. You can change colours using the FG / BG swatch or the colors dock as PixLab shows or bring up the palette edit dock and pick from that. Up to you.
50 second animation. https://i.imgur.com/xHcPyvj.mp4