It was a little dull until I cut through the top to level it off...
The colours are almost neon, and the rings hadn't blurred at all. I hadn't expected it to work so perfectly, and I was pretty thrilled with this result.
I used this butter icing recipe and wound up with an excellent icing, which I spread onto the cake. I decorated with a piping bag, using the black gel colour to turn the icing into about 6 shades of grey. The effect of the dull grey cake being cut to reveal the brilliant contrasting bands of colour was extremely pleasing!
Hooray, Rainbow cake! Happy Birthday, Gail!