I fetched the DROD-RPG source and did a little debugging. It looks like on line DrodBitmapManager.cpp:632
SDL_BlitSurface(pSrcSurface, &src, pDestSurface, &dest);
this seemingly innocent SDL function is converting pixels from { 255, 0, 255, 255 } [RGBA: pure opaque magenta] to { 254, 0, 254, 254 } [almost opaque almost magenta].
But this code is exactly the same in all versions of DROD. So as strange as that is, I'm still not sure why this happens only in Linux, and only for DROD RPG. (And maybe only to users whose CaravelNet ID is "
schep"
.) Maybe there's something particular about the GeneralTiles.png stored within my drodrpg1_0.dat file. Next I'll try extracting that file and see if fiddling with it helps any.