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).

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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 papr, 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.