The plot menu bar in Prime gives three choices for plot background:
How do you change the paper color?
I am not aware of an option to change that color.
There is a hidden menu you open by pressing Shift-AltGr-Q (German keyboard, AltGr-Q creates the at sign @ - so the keyboard shortcut might be differenr on your keyboard) where you can change the color of the cursors and a few other things, but the paper color is not amongst them.
When you open the/a template file renamed to *.zip you find mathcad\settings\presentation.xml. I see font colors there and standard background/foreground, but nothing that I would relate to paper color.
Seems like PTC only dictates the color our paper must have 😉
You set the paper color through whatever color paper you decide to feed to your printer.
I suspect however that if you set the plot background to white, and you feed black paper to your printer, the background of the plot on the print-out will still be black. Same as if you choose the plot background Transparent, or Paper color.
When I am preparing Mathcad Prime sheets for publication I change manual the color of back ground to paper. With screen shot and paint.