Diamond Kote is an engineered product, NOT a PVC, I would definitely use a PVC product like the Azek or similar.

Well that makes sense now. Contractor said PVC if we were planning white.

We were told that PVC would have to be painted if you didn't want white and that a darker color voided warranties and would warp. Truth to this?

Not sure, you would need to check with the company that is being proposed. They do make "vinyl safe" darker paint colors, and I would look into one of them.

I don't think Azek has to be painted, although it does take paint very well. PVC expands and contracts a lot with temperature. Dark colors would increase that. Check the manufacturer's instructions for paint color limitations.

Wood trim and vinyl siding - that is wack!

There are options other than wood and PVC of course. I change out to PVC when I replace wood on our older house. Costs a ton more but is lower maintenance and greater longevity. The times I could see not using PVC is when you want a non-plastic product but you have vinyl siding so that ship has sailed.

If you want darker - than go with things other than PVC