Photoshop :: Why Does Merging [color Shift] My Image?

I'm goofing around with Photoshop 7 and would like to scan in a board of a board game. The board is too big for my scanner, so I have to scan it in pieces.

When I scan the board, it won't lie flat on the scanner (because the scanner edges are not flush with the glass). Instead, it's a bit tilted as it runs over the raised edge, and the part that is tilted up (away from the glass surface) doesn't get exposed as well, so the board image shifts from light to dark as it goes from the part closest to the scan head, to the part that won't quite sit on the glass.