I am very new here and my son and I are thinking of making cookies and decorating them. If I made a batch of royal icing do I just use the food coloring gel that comes in my cake decorating kit?

TIA

YES! same as coloring buttercream

Thanks that is great I have that

And mind how much you put in so as to not change the consistency (if you need a deep color)

If you need a deep color, you should add an extra tablespoon of meringue powder while making the icing to compensate for the extra liquid from adding generous amounts of icing colors. You can add additional powdered sugar to the royal icing to stiffen it back up once you've achieved the color you desire.