As you have shades of red, green, and blue, you also have shades of the alpha channel. The darker the shade of alpha, the closer its value is to zero, and the more transparent that pixel is:
Press Esc to return to the 3D View window. Select the red sphere with the RMB.
In the Properties window, choose the Material button from the header.
In the Transparency subpanel, check the Transparency checkbox and set Alpha to 0.4.
In the 3D View, press Shift + D to duplicate the red sphere. Move the duplicate sphere a bit, so that both overlap, but you can see more than half of each sphere, as shown in the next screenshot.
Select the Render button in the Properties window header.
In the Shading subpanel, check the Ray Tracing checkbox.
Press F12 and the render will look similar to the following screenshot.
Press Esc when you are done looking at it.
