Replied to your post on the 2018 info topic... White Diamond (or "White Frost Tricoat" as they're now calling it) will be available to order on 2018 Sierra Denali HDs this September.
Effective 6/19/2017, the following changes will be made to the Vehicle Order Guide:
Color and Trim section-(G1W) White Frost Tricoat, updated footnote 1 to read "Late availability for HD models. Ordering to begin September of 2017. Available at extra charge.".