I make dark colors with it all the time, and they come out really well, and I have never had a problem with it. The pink and black cake in my photos is all colorflow, and it is really black. I've done brown, red, blue, etc, all really dark and they came out awesome.
I've never had a problem with color either. Just make sure you color your royal icing before you make it color flo. Do not try to color color flo itself. The color your royal icing is, is the color your color flo will be it's just watered. That's how it's always worked for me anyway.
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