3DTin has a color palette at the bottom. It shows 5 most recently used colors and a cube in the leftmost corner that is of the color that is currently chosen.
When user would click on the cube a large color chooser dialog opens (thanks to jPicker plugin), which lets you choose any random color possible. What I did not realize at first that, it was not at all apparent for the users to click on the cube to access more colors. I remember spending long hours perfecting that cube with dynamically set background color, but little did I know that it was in vain.
I had read comments from users complaining about lack of colors in 3DTin. Most of them could not figure that clicking the cube will let them choose from wide range of colors. I had an item on my TODO list to address this issue, but it kept getting side-tracked for one reason or another.
And a couple days ago I went through Google Analytics traffic of this blog. I looked into search keywords that were leading people to this blog. Look what I found.
Users couldn’t have spoken more loudly.
So here is the new design of the color palette.
The icon in the left corner now unambiguously shows that it’s a door to lots of colors.
3DTin 
how to change the background in 3d tin
Good question. You can’t change the background color. In fact there is none. The canvas is clear with paint of opacity zero before it renders your model. So if you export the sketch as PNG file, you can paste it on top of your favorite color/pattern as background (in any third party 2D image editor program)IMO, for a 3D model, the notion of background doesn’t make sense. But if you still want a backdrop for your model, you can create a wall of cubes with color that you want as background and place your model in front of it.