Paint it with a good quality exterior paint (2-coats were it I) and since it is up high and in a guest shower it won't get wet constantly it should be fine.

+1 to the above advice.

I would also, as just a general observation, get a good bathroom fan and put it on a timer switch so that it can be run for about 15 minutes after you shower.

Most bath fans are neither powerful enough, move enough CFM, or are run long enough.

It should also be run to the exterior of the home in this case.

Describe the windows. Are the typical windows double-hung? Is the bathroom window the same? Could it be an awning window? That might reduce the appearance difference.

What's the difference? If you feel the finish color will be too different there are options. General Finish gel stain works great on the Ultrex interior. You can get a very good match and then clearcoat it with another GF product.

Thanks for all the comments. I am convinced we can deal with moisture with the fan and painting. The typical windows in the house are double hung.

Dan - I am more concerned about the look of the windows and not the color. The windows look too different from the outside of the house.

The Integrity series wood/ultrex is unique in that the wood is glued to the interior surface of the fiberglass. The frame of the window is all fiberglass. Since the interior wood could be water damaged, or simply come unglued from the fiberglass, I’d go with a picture window to keeps things simple.