2 boxes of white cake mix
Once I had all of my cake mix ready, I poured all of the batter into my large liquid measuring cup (before I poured the batter in, I zeroed out the weight of the measuring cup on my scale), and noted that the weight of the cake batter was 2 lb. 15 oz., which equaled 47 oz., divide that by 6 (since I wanted roughly the same amount of each color), and you come out to roughly 7.8 oz. per zip top bag.
I did one cake mix at first, because I wasn't sure how much I would need, but it turns out that while I needed more than 1, I didn't need all of 2, but this made exactly 24 cupcakes for me, using about 1 and 1/2 cake mixes.
So, if you choose to use just 1 cake mix, you would need just under 1 cup of the batter per zip top bag; if you end up using 2 cake mixes, it would be just under 2 cups of batter per zip top bag.
Now then, all of the batter has been dived up between the 6 zip top bags, I squeezed out about 1/4 tsp of the food color, in each bag. I wanted a pretty vibrant color, and these colors are called, "electric pink, electric blue, electric yellow, etc", so I knew they were going to be bright! Once I added the food coloring, I just squeezed the bags to mix it all together.
Put the cupcake liners in the cupcake pan, and snip the tip of one bag (you'll want to have a plate on hand, to set the bags on, once the corner has been cut off...). You can see I cut a good sized amount of the corner off of the yellow zip top bag; I shouldn't have cut off that much. It let out too much batter, an instead of having streaks of yellow, it let out enough to have almost a full layer of yellow. It wasn't a big deal, just something to take note of. Unless you're after that look, in which case, go for it!
With the corners cut off of the zip top bags, drizzle a little bit in each cupcake liner; you don't want it to completely cover the previous color, or else it will bake up as a solid layer of color, instead of as a swirl. I also wouldn't recommend swirling them with a toothpick, as you might run the risk of having the colors get muddy. I don't know if that would happen, but I didn't want to risk it.
I had to take a picture of this cute one....I accidentally made a pumpkin face! I couldn't do that again if I tried!
Personally, I love the pink and purple ones! So pretty! However, I think the boys in her class will probably enjoy the ones with more yellow and orange, so to each their own!