I was looking through some of the development and concept photos I've posted and there really seems to be no strict guidelines for color combos. For instance, here's a common promotional photo showing a truck that matches the setup on yours, as in it has white roof and silver nose/bumpers. The grill inserts also appear to be silver.
Some trucks had white roof and white nose. The white nose trucks had black grill inserts.
Then there's the 50th anniversary trucks that had white roof, white nose, and white along the bottom.
Some trucks just had a white nose (my truck is an example)
And some trucks had just a silver nose. Here's one rolling off the production line. Can't get any more definitive than that!
Of course, then all these trucks had options like side trim, eyebrow trim, chrome bumpers. My point is, you could probably find an original example of any combo you want. So if it's originality that you're after, just choose the combo that you like best. Afterall, you could technically order them from Chevrolet in any combo you wanted anyway!