TTO, TTDX and TTDPatch support one of two palettes: DOS and Windows.
OTTD supports both.
A description written by Oskar Eisemuth (eis-os) and Michael Blunck is to be found
here or
here.
Here are the two palette files that can be loaded into your graphics application:
Attachment:
File comment: Windows ttdxpalw.pal
DOS ttdxpald.pal
TTDX_palettes.zip [2.25 KiB]
Downloaded 314 times
And here is a windows palette swatch (yes - that little thing really does have 256 colours

) that can be pasted into a sprite sheet (note there are two formats: .pcx and .png):
Attachment:
File comment: PCX Format
paletteSwatchw.pcx [1.3 KiB]
Downloaded 298 times
Attachment:
File comment: PNG Format
paletteSwatchw.png [ 1.36 KiB | Viewed 2587 times ]