I made up some royal icing and colored it gray, made a frosting plug with it, so it can darken, and
I am seeing these splotches coming up. Kneading it seems to help, but only temporarly.
They keep coming back! What can I do to fix this? This is for a color flow project. A big thanks!

I've noticed that the plastic wrap makes my bc separate when I do plugs - and almost immediately. So when I do any color but white, I make and use the plug as quickly as possible. If it's going to sit a while, I leave it in a bowl.

This problem (usually with b'cream icing) has been kicked around for years. Many people say it's undisolved salt & one needs to dissolve the salt in boiling water before adding to icing - however I found it happened to recipes that didn't use any salt. Others suggest it is undisolved crystals of powdered sugar.
I think it has something to do with the minerals in water. Every community's water is different. Some 'harder' than others; some people use water softeners because of the hardness of their community water system.
Just know that whatever suggestions you get to this ? is really not going to solve the problem