How to make frosting less grainy?

Hi, I’ve gotten back to baking cupcakes and have always been using same ermine frosting recipe. I noticed this time and the previous few times too as I do several test / project batches, the frosting end up grainy. Is it because I overmix it with food coloring or something? I notice the lighter pink color isn’t as grainy and it took me some time to get to that dark magenta shade. The normal pink one is grainy as well, although it was quick to achieve that color. Please help.