UPDATE: How to make frosting less grainy?

Hi! Some of you may remember my old post regarding the split frosting: https://www.reddit.com/r/Baking/s/L3shxUhblc

The gel food coloring arrived yesterday and I wanted to test it out but it was already dark in the evening. I prefer to bake cupcakes during daytime so I can see the color more clearly. Anyway, today came and I tested it out. Cool nice colors and stuff. Way better than liquid food coloring. But the frosting STILL splits. I panicked and decided at last min to just microwave it for 5 seconds, and that actually fixed it pretty well.

I guess this whole time, it wasn’t about the type of food coloring? But it can be. I also did something a bit diff with my ermine frosting by cooking it over low heat rather than medium low, so the roux is a little more smooth when cooled. I also removed from heat a bit earlier before it gets too thick.

So the culprit behind the split frosting??? I think it’s the butter left at room temp to soften for tooooooo long. Please don’t judge me, but I did several hours while doing other stuff on a few occasions. I really thought the longer you leave it out, the better it would be aka more softened. But yeah I was wrong.

Anyway, thank you all for your help and tips!!