Paper for iPad scored a slick color mixer a couple months ago, featuring natural bleeding that you need to see to believe. I use Paper to get ideas out of my head and flex what skills I still have from my multimedia design degree. It’s a fantastic app.
The color mixer itself is impressive, but if you need more fine-grained control to get just the color you want, simply tap it. A popover will appear containing sliders to adjust hue, brightness, and shadows.
Bonus tip: if a white canvas isn’t your thing, you can drop a color from the palette to switch things up.