GWR Carriage roof colour

[Beach Halt]

I managed to get to the UK last week and collected the 2nd hand rolling stock I had ordered last year and sent to my son there.

I now have 3 GWR era 3 Dapol carriages and they all have white roofs. Is that a historically correct colour and how should they be weathered?

I have weathered my wagons and trains but I am wary of over weathering the carriages. Any advice would be much appreciated.