I've been in the pattern of making something special for my brother's two youngest kids on Wednesday when they get out of school and I have to pick them up. Soon I won't be doing this anymore because I definitely won't be able to afford it, but I made them Rainbow Cakes today because at Target it cost me a whole $4, not exactly breaking the bank.
I won't have to buy anything other than MAYBE a jar of frosting next week because I'm saving one and freezing it for next week. :)
You just get a white cake mix, make it as directed, then divide the batter into how ever many bowls as you want colors... then mix in food coloring and dump it all into your cake pans. Don't stir it or mix it after you put it in the cake pans though...it might not come out how you want it to if you do that- unless you just do two colors that are very contrasting (i.e. green and red?). Then bake per normal directions on the box. And then you have Rainbow Cakes! :D