I have a (new to me) tombak that I have used Picreator Metal De-corroder on to remove the rust which was pretty substantial.
The result shows a noticeable difference in color between the core and the pamor.
I intend to try a strong, hot coffee solution (in lieu of warangan) to try to make that contrast even more apparent.
I'm hesitant to post pictures here as it is not a keris.