I made the rainbows by dividing the frosting into different bowls and adding food gel colouring to each of them. Then I piped bands of the colours beside each other on the cupcake (with no piping tip, but if you don't have enough piping bags for each of them like me, you can put them in zip lock bags and cut one end off). Smooth it out with a spatula to blend in the colours a bit. As you can see here, my colours don't look that smooth. In fact, it almost looks like I painted them with my acrylic paints which is most likely because I added a little too much colouring.
I used a cream cheese frosting for this but I think I added too much milk which made it slightly runny. I was also in a hurry to make them and take photos of these beauties so I apologize if it looks weird and not perfect. You could also use other food gel colours to get the rainbow effect or try another frosting like buttercream because cream cheese does not hold its shape very well after piping.
Oh, I am so in love with the glossy colours of the rainbows on my favourite cupcakes.
For the frosting, I used Glorious Treats' Cream Cheese Frosting:
Vanilla Cream Cheese Frosting
½ cup (1 stick/8 Tablespoons) butter (set at room temp about 10 minutes, but should still be cool)
8 oz. cream cheese (directly from fridge)
1 teaspoon vanilla extract (always use pure vanilla extract if possible)
4 cups powdered confectioners sugar
1 to 4 Tablespoons heavy cream, heavy whipping cream (or milk- although I do like the richness that cream adds)
*Place butter in a large mixing bowl and blend slightly. Add cream cheese and blend until combined, about 30 seconds.
*Add vanilla extract and powdered sugar and blend on low speed until combined. Increase to medium speed and beat until it begins to get fluffy.
*Slowly add the heavy cream, a little bit at a time until desired consistency is met. (Don't add too much if you want the frosting to stay in place when piped on cupcakes.)
*Beat until fluffy, about 1 minute.
*Use at once or keep refrigerated. (This frosting will keep well in the refrigerator for several days, but you may need to re-beat it for the best texture.)