DecoColor paint markers work well for touching up plastic parts, playfields
and even small areas on the backglass. The markers are available at art
supply or craft stores. They are opaque, waterproof, permanent; and will
adhere to plastic and glass. They are available in all the basic colors
including metallic gold and silver. They come in several widths. I find
the extra-fine width especially useful for touching up the lettering
on bumper caps and drop targets.