You're right SoD, some colours will harden quicker than others, due to the type of pigment. Enamel paints tend to last much longer in tins, as in the Humbrol and Revell paints for instance. Those paints in glass jars/bottles, will be affected by light, which slowly cures the paint, causing it to 'go off' quicker. If your paints are in glass bottles, which I believe Model Master are, it's a good idea to try to store them in a case or box, in darkness. This will slow the ageing process.