Automatically Change a Cell(s) Colour in MS Excel Windows 7

I'm looking to do a very simple thing in MS Excel for Windows 7. I want to be able to change the color of a 10 x 10 block of squares when someone inputs a value into the top right hand corner. So, for example, this 10 x 10, is sectioned into 4 * 5 x 5 merged cells, or into quarter sections. The 25 cells in this 5 x 5 sections are then merged into one. In each section, one can add a value. I want the code that changes the color of the entire cell (10 x 10 section) into yellow (or blue, or whatever) once you enter in a letter character value. So, in the top right 5 x 5 section, you input 'E', and then the whole 10 x 10 section changes to yellow. If you delete that value, it goes back to white. If you put in 'D', it goes light blue. Then this could be repeated independently across multiple 10 x 10 sections across the worksheet. So, for two 10 x 10 sections beside each other, you could change the value of each. I would need this to work with a total of over 300 10 x 10 sections in a given work sheet.