I’m glad you’ve enjoyed the game! I must admit that I’m not familiar with Roll20. When I have played online, we have used a Discord video chat and a screen share of a graphic design program (Affinity Publisher) where I could easily create rectangles and then change their colors later. If that software isn’t available, you might be able to manage the same effect by screen sharing a PowerPoint window where you can add rectangles over a grid? Hope this helps!