1. Make sure the colors window is open. If it is not, then click on Window in the menu bar and choose colors. The colors window should look like this:
2. Now that the colors window is open, you can left-click on a color to select it as the first color. By right-clicking a color, you will be selecting that as that secondary color.
Don't let that confuse you. Let's have a simple test - choose the bright red color as the first color, and choose the bright blue as the secondary color. It should look like this:
If you select the paint brush and then draw a line using the right button on the mouse (right-click) then the line will show up as the secondary color (blue in this case).
Also, if you were to draw an outlined-filled circle, then the circle would have the first color as the outline and the secondary color as the fill.
TEST - Draw an outline-filled circle with the color settings that we used above (red for the first color and blue for the secondary).
The colors should resemble this: