A little shade seems to help with all colors. Try a light diffuser. I made a simple one by stretching some thin, white, semi-opaque plastic over a 16" hoop.
Plastic shopping bags work pretty well.
I've noticed that the iPhone works better for some colors and my digital SLR works better for others.
Under exposing purples helps to kill the red tones that always seem to show up. Using a gray card or the palm of your hand to set the exposure gives a good starting point.
