Is there a function that tints the *entire room* (background)

[Crimson Wizard]

Have you tried character first? That might be simplier to migrate to since characters are positioned in same room coordinates as objects (while gui is positioned in screen coordinates).

What surface would I use on a GUI,a button? Or just the GUI background image?

Does not matter, but since GUI with button is 1 extra thing, using GUI with background image is simplier.

GUI's also don't seem to preserve alpha channel/transparancy :/

What exactly did you try and what happened?