I had the same problem and was using Amercolor and wilton! No matter how much color I was using, it wouldn't darken the buttercream. I don't think it is the brand of coloring used, but that it is not mixing properly. I think the color had to be added in a different order (but not sure what order). I was going for a dark brown and was using brown and black coloring. I used an entire pot of brown for 1 8'' 6 layer cake - in my opinion WAY more than should have been necessary. I added the coloring as the last step.
As I was icing the cake, when using a hot spatula, the color would saturate with the melted butter! Were you using a hot spatula?
I did think it might have been with the mixing of the color. I add a little at a time until I get the color I need. But when I mix to much my butter cream gets too many air bubbles in it...grrr. No, I did not use a hot knife. I crumb coat first then I put a good amount of butter cream on cake and use a bench scraper to smooth sides. Now this is when it happens. After smoothing, I have to touch up some divots and areas that don't have enough frosting and then the colors are different and I get streaks. Hmmmm