What is the color of a manga paper?
The color varies widely based on multiple factors:


Lighting
Paper type (I'll get there in a minute)
Scanner type and quality


Therefore, I can't 100% give you a color estimate (it could be anything from very light yellowish-gray to very dark gray).



As for paper types. There are also several types of paper that could be used for printing manga:


Newspaper-like paper, which is thin and has a slight gray shade.
Recycled paper, which is yellowish in composure and often has little "grains" in it.


There are probably a whole lot more, but that's the prominent two I know of.