That is probably a burned out backlight. You can get the parts sometimes but on a TiBook (I assume) is doubtful if it is worth the time and effort.
My TiBook a long time ago had the broken hinges, making it unusable as a portable, now this more recent aluminum model PBG4 has a screen issue. So I am now using it as a desktop with an external monitor and a bluetooth keyboard and bluetooth mouse. I use it as our kitchen computer for our iTunes music source utilizing a 500GB external drive for the iTunes library and a set of decent computer speakers. We stream Big Bang Theory and various podcasts for watching during dinner. I probably won't fix the PBG4 screen.
I am undecided what kind of portable would be best next for work, if I need one at all... iPhone, iPad, Air, or PB? I am reevaluating what I expect from a portable. Mostly I am satisfied day to day with my 27"iMac for work at home and serious movie entertainment.