I mean a few overlapped regulag Neo-Geo 4bpp sprites with a same coordinates intended to make an illusion of picture with a colour number per tile more than 16.
You would especially have to worry about the sprite limit (96 cells/line + the total number).
If you are just doing a static picture (20 sprites wide to cover the screen). You can do 4 overlays per sprite. This increases the number of colors to 61 per sprite tile (color 0 in each tile is always transparent, giving you 15 extra colors per overlayed tile). This example means that you would use 80 sprites per line - giving you 16 sprites per line left over (of the 96 sprites per line limit)
In a game, if you used multiple sprite overlays (e.g. 2 per sprite) - you'd have to move twice the number of sprites. But I don't think that is a big overhead at all for the Neo Geo. Usually collision detection takes up much more time and that would still involve the same number of objects.
With a large number of sprites (only for the game example) however, sprite tearing could become an issue - e.g. there is now less time to get all the sprites moved *after* the vertical blank and *before* the display starts but you could do sprite sorting (by y coord) to get around that. I'm not sure how CPU intensive that is because I've never done it myself. But I would guess any kind of sorting is quite slow. Still very possible though on a 12mhz 68000!
