Krud Kutter works great.. spray it on metal from inside a machine that's been in a bar, and you can watch the yellow/brown smoke+nicotene just dissolve and suds off.
I use Greased Lightning with a brillo pad to help remove rust and discoloration from metal parts then use Krud Kutter to remove the grease/dirt.
Never have a problem with either discoloring plastics.
Monitor plexi and marquees I use Novus to polish up after cleaning.