So I mixed up a batch of Antonia's Royal Icing last night and my base designs came out fine. Tonight when I mixed color into it to use it for the details sink holes formed which really messed up a bunch of my cookies. Can someone tell me why this happened and what I can do to prevent it from happening again. Does this only happen with royal icing that is not freshly mixed?
Any help would be greatly appreciated!

The sink holes are air bubbles. I have heard that if you strain the royal icing through a cheese cloth or a brand new knee high panty hose, it will help with that.

Let the royal icing or glaze sit for a few minutes after you mix it and the air bubbles will rise to the top. Give it a stir or two to pop the air bubbles and you will be good to go. If you see air bubbles on your cookies immediately after you decorate them just pop them with a clean straight pin. HTH...