I did that years ago and ended up with five litres of Goblin Green emulsion (latex) paint that was used for my first attempts at terrain and board building. After a few years it smelled really bad but was still useable. This is also why I am careful not to use tap water for any mixing or dilution that will not be used immediately.
I am hoping for an acrylic paint, something between emulsion and model paints in cost and size simply for convenience but you do make me reconsider this, a small tin of emulsion is not cheap but still much more economical than buying acrylic even in 500ml pots. Emulsion also lends itself readily to faffing about – I have often used a mix of emulsion paint, PVA and fine sand or filler, sometimes both, for various jobs.
You are spot on about the tins though, they are not really designed for a lot of use. I ended up pouring the last big tin (of near enough Desert Yellow (Dulux Salisbury Stone 1)) in to a more suitable container.
One thing I did discover was that a paint mixer, the shaped head you put in a drill, is really useful and very quickly resurrected some quite badly separated emulsion that had been sitting around for a few years. This was a trick easily transferred to 1:1 Scale painting which amused me enormously.
A local shop does the paint mixing so I might pop in on Saturday morning.