BS! I have a high flux white LED sitting right here on my desk and it is made from a single die.
Single die with three or more LEDs constructed on it.
High flux LEDs do not have resistors in them either.
Neither do low flux LEDs ( other than constant current LEDs which have current limiting "active" resistors contructed on the die.
And PWM dimming from a current source eliminates the problem of the LEDs spectra changing in a mismatched way until the average current is very low.
Yes because the "brightness" is averaged out by the human eye from full intensity pulses separated by dark. Snag is the eye response is not linear.
You definitely don't change the brightness of LEDs by changing their voltage
You can by varying the voltage applied the the resistor and LED
http://preview.powerelectronics.com/mag/405PET24.pdf