Case : you draw a circle with Red Color, but you need to fill that circle little more white
Circle with default red color
But you need a little more white like this
Here the stupid steps :)
1. Select the object with default color
2. Point your crusor to white box color in color palette (let the circle in select position)
3. Hold "Ctrl" on keyboard
4. Click white box color
5. Yiiiiiihaaaaaa....you can see the change ???
Now you can mix color with the simple way, and you can do that with other color...Try it !! ;)