I am pretty sure Mike is right.
My understanding is that BR imposed a livery requirement of grey tank with red solebars for tanks designated Class A for lighter benzene fuels and all-over black for the heavier fuel oils, designated Class B. This lasted from the 1960s into the 1990s, though I don't know how it originally came about.
The wagon chassis is obviously limited up to a certain weight; depending on suspension, brakes etc.
Because the Class A fuels are lighter, a longer tank is needed to convey the same weight of payload. The heavier Class B fuel oils have the shorter barrel for the same load.
So the longer tanks are grey, while the shorter ones are black. I am not sure, however, why Mobil Charringtons tanks were red! And of course nowadays such colours are defunct; hence the modern VTG wagons we have already done are in a myriad of bright colours!
The light colour for Class A liquids goes back well before the 1960s; pre-WW2, a lot were painted silver, which became a light grey (sometimes described as 'Dove Grey') during the hostilities to reduce visibility to enemy aircraft. The light grey continued in use post WW2. Black was the normal colour for anything with a higher flash point, such as fuel oil, diesel (though this was/is often carried in Class A tanks); unlike the Class A colour, it wasn't mandatory, hence Mobil-Charrington's bright red (Mobil's house colour, IIRC), or Shell's bright yellow tanks for specialist oils. Similar colours were often applied to road tankers; I remember black ones with heavy fuel oil for the local power station, and light grey or white for fuel deliveries to garages.