LED glow intensity definately vary depending on voltage - but the voltage difference between dim and intense is pretty small (you might want to look up the discussion on the benefits of individual resistors compared to a single resistor to power the 4 LEDs in the buttons for the illuminted button hack). The difference in current, relatively, through the LED is much larger - the UI diagram of a LED is a typical diode "knee" curve where the current stays apprx 0 until the threshold voltage is achieved, then current increases strongly for small increases in voltage.

Intensity is related to current, since it's the individual electrons falling through the band gap that each emit a photon. Different colored LEDs typically have different forward voltages (the threshold voltage where the start to emit light) related to the energy (in eV's) of the emitted photon. The photons energy is proportionate to it wavelength (color).

/Michael