It’s an interesting idea and I’m open for adding it. Just note that it would show the tileset-local IDs of the tiles (those that also show up in the Properties window), and not the global IDs (which are only calculated when saving the map). So when you use multiple tilesets on a map, you may see the same number on different tiles.
I guess it could be interesting to use different colors for tiles from different tilesets, though it does raise the question of where that color should come from. Could be predefined like the Wang colors.
I personally try to avoid mixing tilesets within a map so I’m not concerned about differentiating tilesets, but your idea of having the colours like the Wang tiles sounds good! If the colours are associated with the tileset though, then wouldn’t the defaults all end up the same? And if the colours are associated with the map, then the same tileset could have different colours in different maps. I suppose one easy solution is to randomly generate a colour for each tileset when it’s created.