The reason for choice of a light coloured roof may have come about due to heat but was primarily down to cost and efficiency.
The original colour was Limestone. This was used on bodies, roofs, side panels and wheels from the early 60's. Panels being painted before assembly, a standard colour saved the need to hold stock of a range of colours. Ultimately Limestone gave way to lighter shades of white which presumably was a more appropriate match for the wider range of new coloured bodies being introduced.
In Australia, presently only the solid colours still get a white roof and unpainted wheel arches whereas the metallics come with a body colour roof and wheel arches. Given the new paintshop I've no idea why the difference although could still be a cost thing. Personally I think the white roof does a great job of keeping the car cooler and is a must have in Queensland.

