Really? No, really?
Yes, apparently: Really. Instructables community manager mikeasaurus hacked together a plastic soda bottle, the top of an empty spray paint can, and the Schrader valve from a bicycle inner tube to make a homemade aerosol paint container that can be pressurized to 25 psi with a bicycle pump. As the video shows, it does actually work for a minute, or so, but needs repressurizing often.
Nonetheless, a valiant “because its there” sort of DIY project, and even if factory spray paint is easier to use, easier to obtain, and probably even more economical if both time and money are considered, it’s worthwhile to consider what fresh ideas might use this one as a jumping-off point.