I'd airbrush the coloring, starting first on one side with blue and tapering off toward the middle-right; then doing the other side in green, starting relatively heavy with the green and tapering off in the middle-right (but intersecting the previous blue). If you use water-based colorings, there will be natural mixing of the colors in the middle area, leaving you with a blue-green color there. You could either have a circular stencil cut, or do the ring as a transfer. Also, you'd want to block off the purple area while doing the other area; then come back and hit that area with purple while masking off the green and blue areas.
Alternatively, you could paint it, but you may get less uniform blending that way.