Traditional pigments were minerals found with the colour wanted or treated with heat or other processes to produce coloured oxides or carbonates. Simplistically, the different pigments have differing chemical reactions with the gums and solvents used in the paint so the different colours could remain useable for different lengths of time depending whether the pigments are inert or reactive.
Yes, that makes perfect sense of course, but I think you're talking about different usable times for different colours: I'm talking about a situation where I have two tins of the same colour, from the same manufacturer, bought at the same time and bearing markings apparently indicating tey're from the same batch, but which exhibit very differnet usability times. I've found this out because I tend to buy two or more tinlets at the same time for a project, to guard against the possibility of running out before the prject is finished and, in buying further paint weeks or even months late, finding slight batch colour variations. That's what puzzles me: tins of the same colour, from the same batch, with varying lifetimes...
The other factor for useable life of opened tins is whether contaminants have been introduced. Adding some solvents to Humbrol will make it solidify in weeks. I never add anything to the tins to avoid problems.
That's something I'm awar eof too, from talking to Phoenix Precision and from reading Ian Rathbone's 'Painting & Lining' book, so I never mix or add anything in the paint's own tin, I always decant a small amount (using a cleaning metal implement) and mix or add in another receptacle. I collect plastic bottle and yoghurt or dessert tops for this purpose, as they're easily cleaned, used once to mix a small amount of paint and then trown away.
