I have been waiting for the next opportunity to bake so that I could finally try this amazing recipe, and after all of the chaos of my 2nd year of medical school, exams were complete, and there were 3 May birthdays among my group of best friends.  We celebrated our accomplishments and increasing wisdom with this cake!
**Disclaimer: In advance, I again apologize for the lack of pictures for certain steps.  Clearly my instructions skills need to be improved.  Will do better in the future.**
I used white cake mix (nothing from scratch here!).  I had just tried blue and white cupcakes for my alma mater, but the color was not as bright as I had hoped.  After reading a little online, I heard that gel food coloring provides for really bright colors (and doesn't mess with the taste).
I divided my cake mix (made per instructions on the box) into 6 bowls.  To these bowls, I added food coloring to make 6 bright rainbow colors.  For the colors that are not the standard 4 provided, I used the ratios provided on the box or just made it up as I went (not very scientific for someone in the sciences, huh?).
I added these colors into two 8" cake rounds, adding about half of each color into each.  I tried to make the order the same as the rainbow, but starting with different colors in each of the rounds so that the cakes were not just duplicates.  (Again, I just sorta added them as I felt I should).
I then baked them per instruction on the cake mix box.  Then I impatiently waited for them to cool.... the smell is very tempting!
Not wanting to just use plain white icing, I decided to make a sky/powder blue creation by opening two pre-made icing containers, and adding blue until I liked the color.  For this step, you could use whatever color moved you!  It allows for some additional variability in theme, too!  
After icing one layer, I added the next cake, iced it and added some fun sprinkles!  [Note: cake decorating skills also need to improve... still delicious though!]
Later that night, anxiously awaiting the reaction from the crowd, I cut the cake.  Everyone was really surprised how colorful the inside of the cake was-- enough to match the outside.
All in all, I will do this cake a million times over.  And do different color variations for different events, some ideas:
- Another round and Blue&White cupcakes for my alma mater (or any alma mater for that matter.  Get it... mater... matter!)
 - Red, white and blue for Memorial Day or Fourth of July
 - Favorite colors for friends' birthdays
 - High school colors for upcoming graduation season