Coloring arbitrary closed regions
Flood filling is not a very GPU-friendly operation to begin with, and the algorithm described in the paper is not a very good way to go about flood-filling on the GPU either. It might be possible to make it happen with some heroic efforts and compute shaders, but if flood-filling is what you want to do, I would use a different approach altogether. For example this answer describes how to do flood-filling with a multi-pass approach where each pass extends the filled region by 1 pixel in all directions. It will take a lot of passes to fill a large region. You could probably use some variant of jump flooding for a more efficient method.

However, if you don't actually care about flood filling but are just looking to do exercises to learn GPU programming, I'd recommend trying things like: blur filters, outlining, color transforms, image warping, and so on as those will be a lot more straightforward than any kind of flood fill.