Because we are planning a modern design with flat and low slope roofs the roofing of choice seams to be TPO as it has a much smaller environmental footprint than PVC.

The issue with either of these being aesthetics.

With the sloped portion being visible, I am considering covering the roofing with pea gravel similar to they way they used to do it with built up roofs. Other than the obvious weight that has to be considered, anyone and experiance that this would be detrimental? We are in a primarily heating climate (5) but the roof will have full exposure since we are passive solar oriented.

This thought process is not advisable.You are paying for a top of the line roofing material and then you are wanting to damage the product by adding pea gravel.

TPO can be ballasted, according to Firestone: http://www.firestonebpe.com/roofing/ultraply-tpo/systems/ballasted-system.aspx
Pea gravel is smaller than what they recommend. It tends to travel down to the gutters easier than larger gravel.

Yeah, scratch that idea, in order to keep it in place, there would be issues. and ballasted rock is smooth, pea gravel may not be. Will just use colored TPO.