You can create your own colors in My Palettes. You find My Palettes from the Palette menu that you can access by clicking on the circle with colored dots inside it on the lower left corner of your coloring editor.

When you first access My Palettes, you see empty palettes with just one clear circle in them. Tap on this clear circle and the color editor appears. On the bottom you see a row of different types of circles - these are the different color options you can choose from, from solid colors to different types of gradients and finally a metallic color. (Note: Free users can only create solid colors).

Tap on the style you want your color to be. For solids, there is a solid line of color under the rainbow circle which shows you the color you are creating. For gradients and metallics the line has a ball at each end of it - click on the ball that you want to set a color to first, set the color and then click on the ball at the other end to set that color.

The rainbow circle that lets you set your desired color has two wheels. The outer wheel lets you choose the color - slide the white circle along the circle to find the color that you like. The inside circle lets you choose the lightness/darkness of the color. Slide the white circle around to get the desired shade.

When you are happy with your color, click on the checkmark and your color is saved into your palette. You can edit the colors or delete them by double tapping on them to open the editor again, and you can create as many new colors as you want by clicking on the clear circles.