So I made some chocolate frosting tonight for the first time (a butter, cocao, icing sugar, water, vanilla recipe) and it tastes great...

The problem is, it is a lighter brown color and I was hoping for something darker. Is there a way to safely make it darker or might it darken up as it sets?

It should darken as it sets, but if you want something darker, add brown food color. There's also a dark cocoa powder that you could try next time.