Found this one out of sheer desparation 'cos I needed a bit of touch-up paint and was too tight to go and buy some.
When the trusty Halfords areosol is empty - or at least the paint has stopped coming out, press the buttin until the pressure goes. Then insert a sharp knife inot the top of the can at the side and make a small hole. Minding your eyes, let the last of the pressure go.
If you cut two crossed slits in the side of the can at the bottom, with care you can fold back the metal into the can. At the bottom of the can is about 1/3rd Fl Oz of paint guaranteed to match the stuff you have just painted on the boat. I tipped mine into an airbrush jar for storage.
If the can is put on one side for an hour or so, it dries out and can be safely disposed of.
Tim the Wombat