Barb, regular liquid food coloring should work fine. Use red and yellow together, starting with a small amount (a few drops) and increasing to the color you want. Use 1 drop of red to 3 drops of yellow for a nice Halloween orange. To give you an idea of how much you might need,I have a recipe that I regularly color orange for Halloween cookies: for 1 1/2 cups butter and 3 1/2 cups flour, I add 12 drops of red and 36 of yellow. I've never bothered to try to figure that out in fractions of teaspoons - I usually have a little kid or two "helping" me, and we practice counting with the drops. I use a medicine dropper.
Thanks for the recipe - it looks great!