The scrollable parameter input window allows you to mix a "Custom Color." The window is active after clicking on the radio button "Custom Color" in the Selection Panel. To change one of the three parameters to the function glColor3f, place the mouse over the value and with the left button down, drag the mouse up and down. The custom color you mix will be used when you select a new vertex in the display window. This color will remain as the current color until you either mix another custom color or select one of the standard colors.
As with other tutorials, you can also change a selected item (drawn in yellow) using the Up-Arrow and Down-Arrow keys. The amount added to or subtracted from the selected item is fixed within the program. However, a multiplier is also used in making the change. You may adjust the multiplier through the Page-Up and Page-Down keys. Making the mulitiplier larger accelerates the change that is made to the selected item.
As pointed out in the Selection help page, the custom color you mix becomes the current color and is applied to the next defined vertex versus applied to a previously defined vertex. Again, this is consistent to how OpenGL applies the current drawing color.