Ohms Law calculations basically. Just google it, and you'll learn more than you probably wanted to
In a nutshell you do some math that says this is my starting voltage, this is the voltage I'd like to put at the LED, and this is the amperage I'd like this to happen with and viola, you spit out a resistor value.
So, that resistor value takes into account the source voltage which is 3.7 volts, and the forward voltage for the LED at 1 amp which on a red XQ-E is 2.6 volts.
Amber LED would use the same resistors as red
Green doesn't need any
Blue and White can overclock - but a .5ohm 3watt resistor would be what you'd use to keep it safe
That is specifically for the XQ-e LEDs on the SF X4 star and assuming a sound board that puts out 3.7 volts.
Those values are for full output, if you wanted to limit an LEDs power, say you wanted to mix two reds with two blues with the blues at 50% power, then you'd calculate a resistor value using 500ma rather than 1000ma. (in some cases there may be a voltage shift as well, and you'd need to look at the LEDs data sheet to figure that out).