Kerika comes with a standard palette of 10 colors: red, organge, yellow, green, dark blue, purple, dark grey, black, white and light grey: To use one of these colors, click on the appropriate button when you are using the Inspector.
But if you want to get more creative, you can easily mix your own colors by clicking on the big button to the left:

This will bring up up the Color Wheel that lets you mix your own special shades:

The Color Wheel is easy to use: just click anywhere on the multi-colored disk to pick a particular color. Or, if there is a particular color you are looking for, you can directly enter its RGB (Red, Green Blue) values. You can adjust the shade of color by moving the slider on with the graded bar on the right: moving all the way to the top will lighten the color to the point where it will look white, and moving it all the way down to bottom will darken it to the point where it looks black. Finally, you can adjust the transparency by using the scale on the bottom. If you use a transparency value that is less than 1.0, the shape will start to appear translucent. And if you set the transparency all the way down to 0.0, the shape will become completely transparent, whch will make it disappear from sight – although it will still stay on the Idea Page. (Not a good idea, unless you are trying to play tricks on your project team members!)

Want to learn more? Watch a demo on setting up projects.