mWn ist das in der "known-bugs.txt" beschrieben. Problem ist ein Desktop-Feature, welches den Hintergrund nach c.a. 30 Minuten ändert.

Nachtrag:

known-bugs.txt schrieb:

Code:

After a while of playing, colours get corrupted
        In Windows 7 the background slideshow corrupts the colour mapping of
        OpenTTD's 8bpp screen modes. Workarounds for this are:
                a) Switching to windowed mode, instead of fullscreen
                b) Switching off background slideshow
                c) Setting up the 32bpp-anim or 32bpp-optimized blitter