...different color LEDs require different voltages and resisters. Does that factor in to this board when changing the LEDs? Or does it not matter? I seem to remember that red LEDs run on lower voltage then others, like blues, so perhaps there is no danger of over-volting any other LEDs you may swap in?
Great question! The short answer is that it's not a problem in this setup. The current is set by a resistor to 5V, so switching from red to blue LEDs actually decreases the current in the LEDs. The voltage across the LEDs will just be whatever the forward voltage drop is.
But, being a teachable moment, let me give a better answer.
The right way to think about LEDs is that they produce light roughly proportional to the current you run through them. This current then produces a voltage drop. But unlike resistors, the voltage drop is NOT proportional to the current. In fact, it's fairly constant over a wide range of currents. This is known as the forward voltage drop of the LED. Typically it is in the range of 1-3V, with simple red LEDs closer to 1V, and blue (and white, which is really a blue LED with phosphor to give some yellow making it look white) LEDs closer to 3V.
A lot of people have a hard time understanding current and voltage. In the water analogy, current is like water flow (e.g. gallons per minute), and voltage is analogous to pressure, or water height. If you had a bucket with a small hole in the bottom, and you started filling it with a garden hose, water would squirt out the hole. The higher the water level got, the faster the stream would be (due to the extra pressure). At some level, the flow out will equal the flow in. So the faster the flow rate in, the higher the water level will need to get before the system can stabilize. This is analogous to a resistor.
For a diode, imagine that at some height, there is a very large hole in the side of the bucket instead of the small one at the bottom. In this case, pretty much no matter what the flow rate, the bucket will fill up to the level of the large hole. This is kind of what an LED does. (This isn't a perfect analogy, but at least it models the first order behavior.) You can think of different color LEDs having the big hole at different heights. When you connect an LED to a voltage source, this is roughly the same as trying to force the height in the bucket to some level. If it's below the level of the hole, no water, i.e. current, will flow. But it you try to set it above the height of the hole, you would need a massive flow of water as it gushed out the large hole. The point here is that you do not want to connect your LED directly to a voltage source. Odds are that it will either be too low to turn on, or too high, causing massive current flow which burns it out. Oh, and I should mention that the exact height of the hole varies with temperature, etc. So no - you can't just set it to the forward voltage and be done.
So the trick to driving an LED is to control the current. The easiest way to do this is to put a resistor in series with the LED and drive the resistor/LED combination at a voltage higher than the forward voltage drop. For example, if I use a red LED with a 1V drop and a 5V power supply, I will have a 4V drop across the resistor. The current flow through the resistor is proportional to the voltage across it. If I then swap in a blue LED with a 3V drop, then I will have only a 2V drop across the resistor. Thus, the current through the red LED will half as much as the red one.
In the case of the X4 (which I didn't design - I only did the technology it uses), the backlighting is a bunch of red LEDs, each with a resistor in series, tied across 5V. It's a pretty simplistic design (and frankly inefficient design - ~4x more power is wasted in the resistors rather than in the LEDs), but it makes it easy to swap in almost any LED without having to worry about additional power drain. Ones with larger drops will use less current. And because it's running off 5V, you should have enough voltage to be above the forward voltage drop of pretty much any single LED.
If you've read this far, you're probably wondering how the dimming works. Basically, it turns the 5V supply rapidly on and off. To make it brighter, you change the duty cycle so that it's on a greater percentage of the time. So the dimming will continue to work even if the forward voltage drop changes.
This was probably much more than you wanted to know, but hopefully someone will find it useful...
