Lots of options....

Most but not all acid free and museum grade rag boards have the same color all the way through. So when you cut, the inner "core" is the same color as the surface.

Lesser boards (either just cardboard or buffered board) usually have white, cream, or black color inner core.

If inner color bothers you, you may try the best grade available.

Here are examples:

I mostly use 4 Ply Alpha Rag boards, Alpha Mat, and Paper Mat. Former is the same color all the way though. The second has white inner core. Paper kind (which I use for give aways) has cream core.