Thanks for all the lovely comments on my rainbow ones!
A few asked what I used/how I did it. The colour scheme was basically the only way I could think to get 7 colours on 5 nails. I got out a selection of colours, trying to chose ones that were all of similar intensity and brightness. I lined them all up in rainbow order and chose the ones that complimented each other best. From red to violet, these are the ones I ended up using:
Revlon - All Fired Up
NYC - Spring Street
Kiko 279
Opi - Green-Wich Village
Color Club - Chelsea Girl
Barry M - Indigo
Primark - Glo Baby Glo - Ultra Violet
Yes, I know I used a lot of high street brands, but getting the colour right was the most important thing and believe it or not, the Barry M and Primark ones went on lovely. Surprisingly, the OPI was quite watery and the Colour Club was streaky, sheer and dark!
I painted the pinky nail yellow and worked across, green, blue, indigo violet. With hindsight, I should have just done a white base, it's not easy getting yellow to show up vividly over blue and I had a lot of trouble with the blue in general, it came out 4 shades darker than the bottle colour! I painted slightly overlapping stripes of red, orange and yellow on a latex wedge sponge and sponged the pinky, lightly dabbing, moving the sponge up and down slightly and also rolling. I did both pinkies, let them dry and then did another sponged coat in the same way. I did about 3 coats of sponging on each nail, but some needed more for opacity (damn blue!).
Then I went on to the ring fingers. Using the same sponge, I went over the orange and yellow and added another overlapping stripe of green. Sponged both ring fingers the same way as the pinky, avoiding the red stripe that was left from before, letting it dry and then repeating. Then for the middle finger I went over the yellow and green stripes and added the blue. I worked like that until I ran out of space on the sponge and then use another side for the last 2 nails.
Clean up was hell, obviously the polish gets everywhere. You can add some tape around the nail to avoid too much mess. It will look absolutely rubbish and lumpy until you add topcoat! So if it looks awful, don't worry, plus it might need 2 coats of topcoat to smooth out any bumps. If after topcoat it still doesn't look perfect, add a light coat of glitter
I'd imagine this can also be done with gel colour coats or power polish, but do tape the nail to avoid over exposure and cure after each bit of sponging.
I hope that helped everyone! Have fun and post pics! xx