Below is a picture tutorial of what I did and explained step by step. I wish I had some purple food colour to try, I didn't bother trying yellow as it wouldn't make much difference to the chips. I did not add enough green to the water so those chips came out pretty light but still worked a little :)
HOW TO MAKE RAINBOW HOT CHIPS
1/ Get your food colouring together and some frozen chips (you could use potatoes and cut them into chips but this was easy and I didn't have any potatoes on hand)
2/ Fill some cups up with warm water. Add about 10 drops of food colouring and mix.
Drop your fries in each colour and let them sit for about 3 mins. You could use bowls for larger quantities if you wanted, just add more food colouring. The more drops you use the darker the colour will come out. I did not use enough green so those chips did not work very well.
3/ Lay chips out on your cooking tray and put in oven to cook. How long you cook for depends on what the brand is, so check the instructions on the packet. I cook my chips in the oven as its more healthy etc I do not know if this will work in a deep fryer as it may take some of the colour away. Make sure you turn fries over so they don't burn.
4/ Serve and Eat :)
I hope you have fun messing around with this idea with some different colours. Let me know how you go if you try this :) I think these are a good idea for a party or treat but I wouldn't do this to encourage your kids to eat if they are fussy eaters. Just because you don't want to be giving the kids colour additives all the time.