Yeah, that is all my stuff, not the Makespace's. That Teensy LC is overkill for what you are trying to do, but don't worry about that for now. I also wouldn't mind getting it back some day, but don't worry about that either. I have lots of other Arduino-like parts.
The output pins of the Teensy LC provide ~3.3 volts or zero, depending on whether your program tells the output to be HIGH or LOW.
LEDs need a certain amount of voltage across them to turn on. It depends on the colour of the LED. Blue/white need the most, and red the least. They all need less than 3.3 volts, so the Teensy digital output can light them up.
But, if you don't put a resistor in series, if you just hook up the resistor directly, the current that rushes out of the port will be more than the LED can handle, and it may get destroyed immediately.
LEDs typically like about 0.025 amps, or 25 milliamps.
As Peter said, white LEDs need about 3 volts, and the output port of the Teensy LC should be pretty much 3.3 volts. That means if we connect a white LED and a resistor together and put 3.3 volts across them, 0.3 volts are left for the resistor. And we want 25 milliamps flowing from to port through the LED, through the resistor to ground. Ohms law:
voltage = current x resistance
tells us the value for the resistor.
0.3 volts = 0.025 amps x resistance in ohms
resistance = 0.3 / 0.025 = 12 ohms.
The resistor you have in series with the blue LED should already be about 12 ohms. So pretty much what Peter said (20 ohms).
Red LEDs need less voltage than white/blue. Green, yellow and orange are in between.
The (ahem) proper thing to use to make a lightning effect, which only needs a few outputs, and not a ton of processing power, might be the Adafruit Trinket https://www.adafruit.com/product/3500
It is very small, but still as easy to work with as an Arduino.
The Makerspace has some microscopic ATTiny44 (I think) chips, but getting them to work would mean hacking together the circuit and having something to program it. Might be too much hassle.