I ran across this site by accident, so I had absolutely no idea what it is, who made it or why. (I still don't know who made it or why). In fact, when I first got there I had no idea what to do. It looked like an entirely blank screen and I kept clicking trying to see if it would take me somewhere, but all I saw was something that looked like little dark dots falling from my cursor. Eventually I glanced down to see that the dark dots (sand) were falling to the bottom in mounds. This was the result. Kind of ho hum all in black and white.
Eventually I looked around a bit and discovered that This is Sand has been around for several years (maybe you've already seen it) and yes, you can change the colors. There's a tiny little box in the upper left hand corner, and if you click on it, it will tell you that pressing C allows you to change colors and pressing M mutes the sound of the falling sand. Other directions explain how to erase, switch between selected colors and get a gradient. I stuck to simply changing colors, and this was my second effort.
Here's a gallery of designs made by other players. Click on any of them to enlarge the image.