.TPX files are just containers for .TPL files, however, the palette is a little tricky. The first half is a table with Green and Red values, and the second half is a table with Alpha and Blue values. So basically you need to multiply the number of colors by 4 to get the size of the palette data and then rearrange in the correct order.
The trauma center games use a 'tpx' file format that is a collection of TPLs.
According to this post: