Colouring Cells To Show Academic Achievement

I'm a teacher and would like to show progress in my pupils achievements by colouring coding cells in excel. For each subject they are given a level, e.g. 2b. If they show progress, they would move up, i.e. to a 2a. The whole progress scale is shown below:

wc,wb,wa,1c,1b,1a,2c,2b,2a,3c,3b,3a,4c,4b,4a,5c,5b,5a,6c,6b,6a,7c,7b,7a

To make the spread sheet visual i would like colour code the cells depending on whether progress has or has not been made. If they go up (e.g. 2b to 2a) i would like the cell to turn green, if they go down (e.g. 2c to 1a) i would like the cell to turn red. If not progress has been made then the cell can stay uncoloured. Each cells colour would only be dependent on the cell directly to the left.