I don't know if I'm understanding you correctly, so please let me know.
There's really no such thing as a "fixed" palette in SMW. Go to the palette editor
and you can edit the colors of any palette you want. Be advised, however, that changing the palette in one level will often carry over into the other levels, so anything that uses that same palette will also be affected. If you don't want the palette change to be carried over into other levels, make sure that "Enable custom palette for this level" is checked before you start making edits.
I hope this helps.