SirNickity:
I built an indicator out of various 3mm colored LEDs. The red, green, yellow, and blue LEDs were all diffused, but the white was clear. I sanded it with a fine sandpaper and it turned out great. Easy does it though, a little goes a long way. I canned my first attempt because I had sanded several flat spots into it.(Technically, I didn't can it, I just use it for breadboard projects where I don't care about the visual quality. Why waste a good LED?)
At the risk of being a FET pusher, try a small MOSFET instead. This is the perfect sort of application for them. You should use a small (100-200R will do fine) resistor for good measure, but the FET gate (analogous to the base of a BJT) has a high impedance, so there's no need to current limit or calculate gains. You'll need a logic level FET.
I use VN10LPs for moderate current (270mA) -- here's a link to Digikey: http://www.digikey.com/product-detail/en/VN10LP/VN10LP-ND/92610
If you're driving them all at a full 20mA, you'll need something with more gusto. Here's a ZVN4206A good for 600mA: http://www.digikey.com/product-detail/en/ZVN4206A/ZVN4206A-ND/92604
Those are both N-channel, so you'll be switching between the LED and ground. (Same as NPN.)
Nickity, I'm digging this route since, frankly, it seems clearer to me... but that's probably b/c I haven't sunk down into the weeds yet. Is there a benefit/drawback to running these at 20mA vs something lower? It looks like the circuit will all be laid out in the same fashion so there isn't much difference between the two. Would there be any difference in the programming?
Thanks for helping me through this beginning frustration!