If you can't find the exact colour of spray paint you need, or want to use up an almost-empty can of paint, you can make a DIY spray paint can from a soft-drink bottle, a bicycle inner tube and the spray mechanism from a used spray-paint can. You'll also need a few basic tools like a drill, hacksaw and a bicycle pump.
Instructables user mikeasaurus gives step-by-step directions and photos on how to remove the nozzle from an old can of spray paint, attach a bicycle inner tube valve to a plastic bottle, attach the spray nozzle to the lid of the soda bottle, fill the bottle with old paint and pressurise with a bicycle pump.
The author instructs that you should only mix like types of paint, but you could probably get interesting effects by mixing oil and latex or acrylic paints. The author also recommends adding one part water to six parts paint to make the paint less thick, but that is probably why the paint in the video above drips so much. Again, experimentation here will probably help you nail down the result you're looking for.
DIY spray paint [Instructables]