Ideas in Excel

There are probably a lot different tools for drawing 16×16 images but I found a quick solution that works surprisingly well.  I just use a standard spreadsheet and multiselect/set all the columns to squares.  After that you can easily change the colors of each cell using the cell formatting “paint can”  If you want to change a bunch of cells at once, hold the CTRL down while clicking the cells you want to change.  This will allow you to select them all.