need color inversion script for osx on older mac

A good friend of mine had a stroke a few years ago and it affected him such that he is sensitive to glare.. which is why it would be great if he could do what I do-- use a keyboard shortcut or an applescript to invert the colors so it's all white text on black instead of black on white.

Trouble is the applescript I use doesn't work on his machine and neither does the keyboard shortcut. One has to toggle back and forth depending on whether one is reading or viewing pics or vids, and it's a pain to have to open system prefs then unaversal access, then hit the button and then close it all every time one want's to toggle.

We both have the same os, but I use an imac and he has a powermac g4. Maybe he's just sol cuz his machine is older, but maybe someone knows a workaround or has the right applescript or something.. ?

I don't know diddly about writing scrits myself, but I was lucky enough to find one on the web that works for me.