This is the first in a series of color by number games I’m designing in Microsoft Excel. The worksheet uses conditional formatting to change the fill color of specific cells based on their contents. Then I add a simple formula (=[cell number]) to link the indicated cell to the place where the user types in the answer to the addition question. If they get the answer right, the cells change color, gradually making a picture.
This one is super simple, just four addition questions that yield the image of a Goomba from Super Mario Brothers ™.
Any kind of pixel art is easy to do; you just have to count how many pixels high and wide the image is so you know how many cells in the worksheet you need. Then resize the cells so they are square and go crazy with the conditional formatting. It’s a good idea to go into File — Options — Formulas — Error Checking Rules and uncheck “Formulas inconsistent with other formulas in the region”, otherwise you’ll get unsightly little green triangles in the corners of the cells in your image.
Chronicler's Note: 
I never thought of using Excel in this way. Thanks for sharing.
When I worked in a bank, one of the tellers I worked with would program these huge, elaborate spreadsheets this way with all kinds of interdependent calculated cells. Then he would just plug in random numbers and create constantly evolving abstract works of art. You have a lot of free time on your hands when you’re a bank teller.