Yup - because white is a mix of all colors, while a LED typically emits light heavily centered around a specific color/wavelength...

I've seen pretty decent white (not high-intensity though) from RGB LEDs (LEDs with 3 (or more) diodes internally, one each red, green, blue), but it required some finetuning of the voltages to each element...

One can also do pretty neat effects with independent slowly changing voltage on each of the inputs...

/Michael