Being a tile based puzzle game, you'd want the difference in tiles to be easily defined (eps. for kids), although a nice transition would look more visually appealing.

I like the idea of multitexturing, and just changing the vertex alpha value, wouldn't take much at all to implement. Though what about limit on TextureUnits? In the end there may be more than just 3 different tile graphics per level. Or using a different method?

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I'm quite pleased on how the artwork is coming along so far, and in my style of rendering out pictures when the art isn't really finished yet, here's a quick little pic I just made up:



Still not finished at all (Will eventually have all the game characters), just an idea for the splash screen, or a banner, or something