White strain???

I'm growing some white lightning and I just noticed that the very top of the buds are sort of yellowish. Like the interior catylists are growing that color. Is this just because its a very white strain or something else? I have six total from clones, sog. the plant looks great 12/12 for 3 weeks.

or maybe the light is too close. its a 400w hps in a cool tube,I can touch the glass no prob its not hot but it is like 4inches from the top of the buds