I bought several fiberglass windows for my father's commercial building, and it was the worst decision I ever made. The problem is that fiberglass is not "tough" the way wood or PVC is, so when you drive a screw into it, you break the glass fibers around the hole, they don't move this way or that to accomodate the screw thread the way wood or PVC does. So, the screw holds initially, but with the wind buffeting the door, you can count on those screws working loose, and then you can't tighten them without breaking more glass fibers. You can't even take the screw out and put it back in without risking the possibility of cutting a new thread through the fiberglass, and that accelerates the stripping of the screw holes.
So, if you still want these doors, check to ensure that they have solid fir or a hard wood like ash in those critical areas where you need to drive screws (at the hinges and lock set). Otherwise the screw holes simply strip, and you're left to your own resources to do what you can. With the fiberglass windows we bought, we were actually using 4 inch long deck screws to hold the window hardward in place... the holes in the fiberglass were stripped, so we ended up using longer screws and driving them right into the original wooden window frames to get the window to open as it was designed to. I will never buy another fiberglass window again, and I would urge you to talk to some people that have owned fiberglass doors for 10+ years and hear what they have to say.
There is absolutely no problem painting PVC. The only reason window manufacturers make all their windows in WHITE PVC and recommend against painting the windows is because PVC softens up when it gets really warm. The idea of making the windows out of white PVC is to reflect as much of the Sun's light as possible, thereby keeping the PVC as cool as possible. But, any latex or oil based paint will stick well to any colour PVC.
The reason why staining fiberglass doors won't work is because stain works by differential absorbtion into the more porous parts of the wood. The growth rings in wood contain smaller wood cells than the wood between the rings, and that results in the wood being less porous at the growth rings. When you stain the wood, MORE stain is absorbed by the wood between the growth rings, resulting in this more porous wood being stained a darker colour than the wood at the growth rings. This is how wood stain "brings out the grain" in wood.
Fiberglass might have a surface texture on it that resembles wood, but the fact that it isn't porous means that you won't have any stain absorbtion, and the stain will simply wash off from rain or wear off from handling or wear and tear. Also, there's no grain involved, so what you're going to see won't look like stained wood. If you do go ahead with $%$*&#ing fiberglass for your door, then just paint the PVC jambs with an exterior oil based paint whatever colour you like. Since there won't be any screws in the jambs holding up any serious weight (as there are in windows), the necessity to keep the PVC as cool as possible doesn't really apply to door jambs.