Hi!
I have two Kobo Glo HD's and they're very different. One is very white and one is yellowish, as in the white one is 100% white (like really clean paper, almost RGB 255|255|255; and the other one, - slightly yellow, 249|248|236.
Here's a picture comparing those two colors:
I also asked my mom and dad to confirm that I'm not crazy and they said, indeed they're very different. Dad called it as "this one is LED, and this one is incandescent".
So, which colour is your Kobo Glo HD?
EDIT: (I didn't notice a difference when the LEDs are off. I think the leds are different color.