Don't use
startcolor at all.
Only add the setting if you plan on recoloring a different shade of color, but since you clearly don't want to do that but instead want to recolor the original green shade, just remove the option.
Also, make sure that
prefcolor points to an actual color in the game.
skincolor is not a valid option in S_SKIN.
Check the S_SKIN page in the wiki for more details.
Also, triple check that every shade of green in your sprite matches the palette indexes 96 to 111, as that is the range the game will recolor.
Note: Check not visually, obviously, rather make sure that your spriting application has the option to use a palette and use the SRB2 2.2's palette, or just compare the RGB values of each green with the palette equivalent.
Here's the palette.