PS: Yellow roof or bonnet were on vehicles which spent their lives on the airfield...
Many thanks for the suggestions and pictures.
I'm aiming to tell a story, rather than present an accurate model of reality, so I have a little leeway in colours and designs. Apart from anything else car I'm using is actually based on a Russian prototype; I'm using it just because it was the only police vehicle available and I like the shape.
The story (actually a tabletop game) itself is set on ascension Island in a world where WW1 ended in 1917; Britain is now heading toward a civil war caused by King Edward VIII refusing to abdicate; and airships are the main form of international air transport with heavier than air planes coming a distant second. In this context I try to keep things similar so they're recognisable, but just a little different. In this context I need 'markers' the people will recognise easily. There also needs to be a good reason for everything for it to be believable, for example, in the context of an aerodrome primarily using lighter than air craft, I can imagine that painting roofs white or yellow will quickly catch on to avoid any thing choosing the patch you are occupying as a landing space.
Having looked at the pictures above (and my goodness I'm old, I can remember our family Chevette) I'm going to have to think a bit before committing to a design; I'll probably try and combine aspects of those with other ideas.