Add a little bit of black colorant (I used black oxide) in with your favorite red. I never got a perfect cranberry color, but it was close enough (sorry, I don't have any photos). Add just a bit of black at a time. Hope this helps!
I have trial by fire, red raspberry and the first custom red mix Carrie made. And black mica. I'll see about dark blue too. I'm planning a candy cane shop so I should just make an extra 4 ounces of batter and test colors.