I jumped on the rainbow wagon to make this delicious cake for a friend a couple months ago. (Yes, a couple months ago — sorry, I lost the pictures till now. Geez. Back off.)
My friend, Julia, gets super excited about rainbows and colors in general, so I surprised her. I figured it would be a lot of work, so I did cut one pretty big corner: I started with a white cake mix. I mixed the batter according to package directions, and then divided the batter into 6 bowls. Then I dyed each bowl with gel food coloring, mixing a healthy dollop into each bowl until it looked like the right color. (Use gel instead of regular dye — you won’t need to use as much, so it won’t water down your batter.) Though I’ve seen neon, which looks pretty cool, I went with traditional ROYGBIV colors. Except I combined indigo and violet into one color, since they’re pretty much the same anyway.
After I mixed each color, I poured each layer into a bundt pan one at a time. (And yes, I did it in rainbow order.) I tried to pour it relatively gently so that the layers wouldn’t mix. But I didn’t worry too much about it because I wanted them to swirl a little to get a tie-dye look.
I set the cake to bake according to package directions, and while it was in the oven, I got started on the strawberry buttercream frosting.
The cake passed the toothpick test, so I pulled it out to cool for 10 minutes or so. Then I flipped it out of the pan. I noticed that it didn’t rise very much in the oven. I probably over-stirred it while trying to get the colors mixed well. So word of warning — stir the colors in, but try not to over-stir.
After I frosted it, I thought it looked a little plain for something so super fantastic as a tie dye rainbow cake, so I added rainbow sprinkles on top. DOUBLE RAINBOW.
ALL THE WAY.
I really love that the end on the right almost looks like a peace sign. I’ll just pretend I did that intentionally.
The frosting really takes the cake (ha), so if you didn’t want the hassle of a full rainbow, you could just use any white cake recipe you like. Enjoy!
Wine & Plum 
THAT’S MY CAKE!!!
This is such an awesome recipe! It’s also great for baking in mason jars so you can see all the colors! :)