Not 100% sure this is all accurate, but this is what I think:
You have a file with the extension .lua, and you included a lua script as a lump *within* that file as if it were a WAD. When SRB2 adds a file with the extension .lua, it assumes it's reading a plaintext file (as if you saved it from a text editor, not SLADE), so it gets confused when it starts reading WAD header data that SLADE saved.
If you originally created this file as a "Wad Archive" in SLADE, try just changing the file extension to .wad and see if it works.
If you originally created this file as a "ZIP Archive" in SLADE, try making a folder called "Lua" in the archive, put your script in there, and then save it with the extension .pk3.
If you don't intend to package anything else with these skincolors, though, you really should just make a proper .lua file! Copy your script into a plain text editor (like notepad.exe), and save it with the extension .lua.