I forgot to post yesterday, doh. It's coming along rather well though.
I added in palette swaps today, I'm doing it via directly changing the pixel data of the texture used as it hasn't proven to be slow or anything (the texture is only 256x256). The original colors, and my attempt at dithering there, were to try and make it look like silhouette on a sunset painting sort of thing... It doesn't really look like that, so the default palette will likely be different.
I've added in button states, so it's clearer to the player when an upgrade becomes available.
I'm going to be either making the cost boxes bigger, or tweaking the small font slightly as the text doesn't quite fit in there snugly when you hit the millions (M is always a big letter!) But the whole point of these idle clickers seems to just be about racking up those numbers into the quadrillions. :)
Day 3+4:
