So far, I have the rainbow plus pink, white, and silver. What other colours should I do? The polish is, for example, red(or any other colour) with the cracked polish on top in black.

If you would like feedback, you should post pictures of your project, otherwise we can't give you any advice

I can't until I figure out how to take decent pictures of it ><
However, the style is somewhat like this;

For in-game shots, have your sim wear the nails, turn off free-will, have the sim pose that shows the hands, then push "Tab" button so you can get really close and make sure to put the texture on high-- there's a couple of tutorials on how to do that Link and Link. If you were talking about getting good bodyshop pics of the nails, then I don't know you probably can only do the nails in-game..

I prefer in game since bodyshop takes forever to load. Thank you!