Most of that information comes straight from the FAQ with the exception of the yellow cap which I included for completeness.
I think Volvo started using the higher pressure grey/green cap on later Turbo cars, presumably to increase the boiling point of the coolant in response to greater heat output from the Turbo. Because the earlier 140/240 systems were not designed for the higher pressure the new, high-pressure caps are not backwards compatible. Apparently, using one on a 240 can damage the radiator. People do swap to the black cap on 940s to mask problems in their cooling system. If you keep the cooling system properly maintained you should have no difficulty using the cap Volvo intended.
The coolant reservoir and cap are on the side from which the pump draws coolant, so the pressure could get a bit higher on the downstream side of the pump.
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