The 3pi robot ships with two through-hole red LEDs and two through-hole green LEDs. There are connection points for three optional LEDs on your 3pi: one next to the power button to indicate when the 3pi is on and two user-controllable LED ports near the front edge of the robot. Using these LEDs is completely optional as the 3pi will function just fine without them. You can customize your 3pi by choosing your desired combination of red and green LEDs, or you can even use your own LEDs if you want more color/brightness options.
Note that you should only add LEDs if you are comfortable soldering, and you should take care to avoid desoldering any of the components near the through-hole LED pads. LEDs are polarized, so be sure to solder them such that the longer lead connects to the pad marked with the +. Before you solder them in you can press-fit them in place and check to make sure they light as expected. Once soldered in place, carefully trim off the excess portion of the LED leads.
Your 3pi also ships with three shorting blocks of each color: blue, red, yellow, black. This means you can customize your 3pi by selecting the shorting block color you most prefer, or you can use a mixture of colors!