This technique is something I saw on the internet a long time ago. It is simple, unpredictable and a lot of fun.
I lined a container with foil. It makes it easier to clean up. It has about 1cm (1/2 inch) of water in the bottom. The paper towel is for the wet cards. You need a dark colored card stock. Here I have painted an index card black and gold. You need clear nail polish, the cheaper the better. Some of the expensive brands won't work. I have tried it with colors but it really only works with clear polish but you can experiment with whatever you have.
Place your card in the water and push it to the bottom. It may take a minute to get it to stay there. Using the brush in the bottle let a drop of nail polish fall onto the water. It should immediately spread out into a rainbow of colour. That is all there is to it. Use tweezers to slowly pull the card out of the water and the nail polish will stick to it.
I laid the cards on the paper towel to dry naturally. Be careful not to touch the polish until it is dry or it will come off and stick to you instead. When it dried the polish over the painted area did not look bright enough so I glued the cardstock on top instead.
I use a paper towel to clean the surface of the water if I want to do another card. You can see the left over polish. When I have finished I pour off the water and then put the foil in the bin.
This is what the technique looks like when viewed from straight on. You can see some of the color but when you move the card in the light you get the full effect you see below.
See, I told you it was easy. Have fun an experiment with what you have at home.