Hi Ken,
I am not sure what would be the easiest way to go about it. You use your checkered board to look for the green columns (or rows), but you also look at the Total row/column to see which period came out best.
I don't quite know how you get the percentages in the table,
but when you use for instance Yahoo closing prices, just put [February close]/[January close) in the box for Februari. The colour should be green when this number comes out above 1, below 1 gets the pink. For the total you don't use SUM(), but PRODUCT(). I think that should do the trick.
If you like, you could do all this somewhere in the background and still use the old percentage gains in the boxes. You just don't calculate the Total from the boxes, but pull it from the 'background'. (And you may even change it to percent gain then.)
And Gummy Stuff that QP mentioned is a very nice site! Highly recommended!
Regards,
Karel