Given that users are turning their Porters inside out for packrafting, the obvious solution to the heat issue is reversible packs, heat absorbent black on one side, and either white, or ideally reflective silver, on the other. With detachable shoulder straps and waist belt, and fittings either side, the user can turn them one way, or the other, depending upon the environmental conditions.
That at least takes care of the extreme conditions, admittedly at the cost of extra weight, but fails to deal with the middle range.
I suspect the only satisfactory solution is going to be a smart fabric, that adjusts its color, reflectivity and even permeability to the conditions, either automatically, or under the control of the user. Somewhat like an octopus or squid changing its appearance to match its background for camouflage purposes.
Such a miracle fabric would more likely be developed for tents, before packs, and would also incorporate (controllable) ventilation. It would need to be combined with an environmental sensor (or sensor network). Plants' responsiveness to their environmental conditions would be a good model.
In I think the 70s, there were interesting membrane walls developed for pneumatic structures that controlled the transmission of light and heat, and the opacity (something akin to Steve Baer's experiments (for fixed buildings) at Zomeworks, see http://www.zomeworks.com/about-us/ ). The pneumatic membrane walls were controlled by pneumatic systems inside a double wall structure (Baer developed Beadwall moveable insulation - double glazed structures with polystyrene pellets that were sucked or blown into window cavities to provide insulation when needed, then voided and stored when not).