Born Pretty glitter flakes
This is a very simple and versatile look using glitter flakes from Born Pretty. They come in multiple different colours, and you can add as much or as little as you want depending on how opaque of a look you want. You need a black base, but no gel or other special sort of nail polish is required!
This is what it looks like straight out of the box. It's very hard to tell the difference between the different shades before application - I have 9 different shades and I struggle to see the difference before putting them on my nails.
The colours turn out very bright and opaque once applied over black nail polish, though.