Here is a quick DIY to revamp your old, plain phone cover. All you need is a plain phone cover, any colour will do, tons of nail polish in different colours (metallic and plain) and a top coat.
Load your nail polish brush up and then just swipe it onto the phone case in multiple directions. Layer different colours for some dimension. Continue doing this until you are satisfied.
Once completed, let the nail polish dry for about 1 hour before putting on a layer of top coat to seal in the design. After painting an even layer of the top coat allow it to dry for 4-5 hours to ensure that it is all dry.
This is the completed design. I'm really happy as to how it turned out.