How to Make Your Own Nail Polish Color
To make your own nail polish color, start by choosing two nail polishes that you want to blend together. Try to choose colors that will make a totally new color when you mix them, like green and yellow or blue and red. Then, pour a little bit of each nail polish into a plastic cup and stir the different colors together until they're fully blended. If you're satisfied with the color, paint your nails with the new polish or carefully pour it into an empty, clean nail polish bottle to use later!
Select eyeshadow in the color of your choice. Pick eyeshadow that you don't plan to use again. You can make nail polish from any color eyeshadow you want. You could even mix a few eyeshadow colors together to create your own custom shade. Crush the eyeshadow in a Ziploc bag. If your eyeshadow is a loose powder you can skip this step, but the solid blocks of eyeshadow need to be crushed. Put the eyeshadow into a Ziploc bag and crush with a spoon, the handle of a makeup brush, or a rolling pin. Crush until it is a fine, consistent powder with no lumps remaining. Be diligent with this. If it's not smooth when you dump it into your polish, it won't mix well into the clear polish. Take care not to puncture the bag. Snip off one corner of the bag. You want a tiny opening, so make sure you only cut the very end of the corner off of the bag. Alternatively, you could make a small paper funnel and pour the eyeshadow from the bag into the clear nail polish using the funnel. Pour the eyeshadow into a bottle of clear nail polish. Open the bottle of clear nail polish. It should be three-quarters full or less to make room for the powder. Position the cut corner of the bag in the neck of the bottle, but take care not to dip it in the polish. The amount of eyeshadow you add will determine how rich and dark the color is. Add a little shadow at a time until you reach the desired shade. Alternatively, you can use white nail polish to create a more opaque color. Replace the cap and shake the bottle. Shake the bottle until the pigment is evenly distributed. To keep the ingredients from separating, shake the bottle regularly, especially before you use the polish. If possible, purchase small, steel mixing balls or ball bearings and add 1 or 2 to the bottle. Mixing balls or ball bearings help mix the polish in addition to increasing its longevity, and you can get 100 of them for only a few dollars online. Some shadows will come out very matte. If that's not your style, just add a clear top coat on your nails to make them shiny. Choose at least 2 colors of nail polish. You may want to choose cheap nail polish in case the color doesn't come out as you envisioned. However, you should mix polishes with similar formulas, such as 2 colors from the same brand. Choose colors that you know will mix well. For example, mix a silver sparkly polish and a dark purple polish. Alternatively, make an orange polish richer by adding a little red and yellow to it. Pour a small amount of 1 nail polish into a small plastic cup. Begin with the darker color and add the light color to it. Alternatively, you could use a paper plate as your paint palette, but it will be harder to transfer the paint to a nail polish bottle. Add a little of the next color to cup and mix them together. If you are mixing more than 2 colors, add the other color(s) as well. Use a nail polish brush, paint brush, or a toothpick to mix the colors. Adjust the color by adding more of either color to the cup and mixing again. You can even add glitter to the polish for a sparkly touch. Store the new polish in its own bottle. Pour the polish from the cup into a clean, empty nail polish bottle. If you have them, put 1 or 2 small, steel mixing balls or ball bearings into the bottle to help mix the polish before painting it on. Choose a kit. Several companies, like Bramble Berry and Mixify, now have “make your own nail polish” kits that come with colorants, glitter, nail polish base, and supplies for doing the project like a scoop, funnel, and palette. These kits cost between $30 and $40 each. Select a kit from a beauty supply store or order a kit online. Pour the colorant into the palette. Open the containers of colorant that you want to use. Pour a small amount of the chosen colorant powders into separate depressions in the palette. Open a nail polish bottle and place the funnel in the top. The kit will have several nail polish bottles labeled “mix” (or something similar) on them that you use for this step. A small funnel is also included. Add the colorant, replace the cap, and shake the bottle. Use the provided scoop to put your desired colors into funnel in the bottle of nail polish base. Then, remove the funnel and replace the cap. Thoroughly shake the bottle to mix the colorant into the base. Adjust the color, if desired. If you aren't happy with the color, remove the cap and add more of the colorant you want. Replace the cap and shake the bottle again. Keep adjusting until you reach the perfect shade. 
Select eyeshadow in the color of your choice. Crush the eyeshadow in a Ziploc bag. Snip off one corner of the bag. Pour the eyeshadow into a bottle of clear nail polish. Replace the cap and shake the bottle. Choose at least 2 colors of nail polish. Pour a small amount of 1 nail polish into a small plastic cup. Add a little of the next color to cup and mix them together. Store the new polish in its own bottle. Choose a kit. Pour the colorant into the palette. Open a nail polish bottle and place the funnel in the top. Add the colorant, replace the cap, and shake the bottle. Adjust the color, if desired. 