Mainly radar calibration, yes. Impressed ones went into RAF camouflaged colours, IIRC, with yellow undersides. There’s a famous, good shot of one at Duxford during the war with the radio aerial fitted, and an example on show there now, without. I could imagine civilian examples turning up in pre-war schemes until camouflaged (1938/9, prob not 1940, though). Civilian examples were painted in cream, red, and blue among other schemes, but always with big G-xxxx and usually cheatlines in contrast.

Hope that helps, I’ll have a dig about for further gen…