When I started out with XNA, this was my learning project. My goal was to create a tower defense game with nothing but colored squares - no fancy sprites (hence the title). I tried out a lot of things, learnt a lot, but ultimately I shelved the project. I've put it on here because I thought maybe someone who's learning XNA (or, more likely these days, MonoGame) might be interested in it as a simple example for this style of game or the ingame map editor I made.
I should probably point out that this was some years ago, and this isn't really reflecting my level of skill and knowledge anymore, and I would probably do a lot of things differently today. It's like they say:
"If you don't hate your code from half a year ago, you haven't learnt enough."