Paper has this rotary mixer that allows you to add "paint" and mix new colors. Turning the dial adds or subtracts the color you have picked. It's a brilliant interface that's imagined to be used with touch. The mixer is a monumental improvement from fiddly sliders you normally see in competing apps. When I was first sent a screen shot of the design, I didn't think much of it. But using it makes you realize the beauty of the design. It's so natural. I've also found myself trying out new drawing styles with the addition of color. It enables you use almost gouache-like techniques. It's so much fun.
Paper has also added a palette for all of your custom colors. Color for Paper is an in-app purchase at just $1.99. Highly recommended for anyone that uses the app, it's brilliant.
Here are some sketches I did recently using colors. For more info.