When do you put the coloring in? If the butter cream is not finished (emulsified), it may not be able to incorporate the extra liquid from the food coloring. That might be the problem, although I suspect at least some of your attempts have been without coloring. You did not mention your recipe. It may simply need more butter. The IMBC recipe I used to use that worked well was 2 cups (1 pound) of butter, 1 cup of sugar total, and 5 large egg whites. One time I made IMBC and it looked just like yours. I added another cube of butter and it came right together. I assumed I had counted my butter wrong. Try adding another cube of butter, and it may come together. I used to make nothing but IMBC until about a year ago I gave SMBC a try. I have never gone back. It is easier to make, does a better job of pasteurizing the egg whites, is less buttery tasting, and less finicky. There are lots of recipes on CC. Here is the one I use.
SWISS MERINGUE BUTTERCREAM
10 egg whites (1.5 cups)
2 cups granulated sugar (14 ounces)
24 ounces (6 sticks) unsalted butter at room temperature cut into large chunks
¼ cup vanilla, or other flavoring
*Mod edited to remove mixing instructions.
I hope that helps. i wish you success, because meringue buttercreams are so good!