You can't change their brightness and mix various amounts of different colours to get the full colour palette.
So you can have only 7 different colours of notification LED (2^3 = 8 which includes all three LEDs off - black).
ff0000 is the same as 010000 - red LED on.
You have to make sure that at least one of the RGB values is zero, otherwise it means all three (R & G & B) LEDs are on = white light.
ff0000 - red 00ff00 - green 0000ff - blue ffff00 - yellow ff00ff - magenta 00ffff - cyan ffffff - white