Touch up paint is about £4-50 from your local overpriced VW-Audi dealer. I bought some for my Dolphin Grey A2. It comes with lacquer. Depending on how careful you apply it you can get away with covering minor stone-chips and scratches (I use the end of a cocktail stick followed by a thin cutting compound when dry), but this is probably no substitute for a chip-away type repair. But then again these can cost quite a few £10s. The aim of the touch up paint is to hide any exposed primer if this is a different colour from the top-coat, or if the chip has gone through to the metal, to prevent any corrosion .. and yes Aluminium does corrode.
Tino
Dolphin Grey 1.4 SE