well Susan this I can answer with my experience.
I mainly use buttercream, but I have also used whipped icing, and store bought icing and colored it, and the color does still darken.
For the spiderman cake I did last year I put alot of the Wilton "no taste red"
let it sit and it did darken,
I added more let it sit and it darkened more.
So in my experience yes it will darken more and more
..... are you using Americolor or Wilton coloring? like I said above I use Wilton right now, and I will buy some Americolor colors once back in the states.... it supposedly gives you a deeper brighter color vs Wilton.... so try that if you can... I know alot of people on here use Americolor....