Kimmo, you could darken / discolour brass with ferric chloride, which is used to actually do etching. You can buy that in an electronics supplier as it's used for etching circuit boards. It can be painted over just the same as brass as it leaves the metal *very* clean. Different strength mixes and different times produce different effects - not sure how easy it is to control though, so a product like this may be more predictable. Interestingly, considering Darren's comment on crystals around the bottle top, ferric chloride can be bought as crystals which you then mix with water - I'm not saying that's what this is however...