Ventilated vans were used, and the fish were carried in long flat wooden boxes, packed in salt. While I cannot put my hand to any photos, they were like standard goods vans of the day, with slats at the upper or lower parts of the sides. Fish were also sometimes carried in "soft-tops" - convertible goods vans with the tarps open on the roofs.
Livery - soft-tops - grey, standard wagon livery. Ventilated vans, depending on type or railway company, could be the standard wagon colour, or possibly a coloured livery - often that of carriages on the same line. I have a notion that MGWR fish vans were at least at one time a lighter colour to reflect heat, but I'd have to look up details. Since my house move there's a load of stuff I just cannot find!
On the GSWR, you're safe with the wagon livery of the day, a very dark grey, sometimes almost black - not unlike what locos would end up having from 1915 onwards. I cannot say for certain whether any GSWR vans carried the passenger dark lake, but I wouldn't be surprised. Do you have any drawings or pics of such a van? If so, let's have a look at it..........