Quote - the material room preview, do you mean the lil window on the main node?  the lights in use effect the preview. 

i had a purple light, the material thumbnail saved to my lib had a purple tint  but, of course, next time i used it, it wasn't purple.

Yeah... the preview doesn't seem to work with a lot of the newer nodes etc... EZMetal or SSS shaders etc, just shows black... or shows the texture without the effect from key nodes...

...although pretty sure this is an issue with OpenGL (and maybe also software preview?) preview, full stop... rather than just that mat room preview window?