It's a fad thing, not an economic thing.

People build to what may be trendy and then spend thousands trying to offset that heat buildup with air con and then complain that the power bill is too high to pay.
It's all designed to generate market sales so do people really think about what they are building or buying.

The only reason industry went away from the old galvanized roof in the first place was it was too glarely for the neighbors and then came the red and green painted roofs, in qld anyway.
I like reading on some history on these basic issues.

History of COLORBOND® steel

Long before COLORBOND® steel there was corrugated iron. Way back in 1843 in England, it was discovered how corrugating a thin sheet of iron could produce a very strong, lightweight building material.

Within a decade, John Lysaght brought corrugated iron to Australia, where it was quickly adopted by the early settlers as shelter from the harsh conditions of their new environment. Lightweight and stackable, it was easily transported across great distances, even on camel back. In some cases, complete prefabricated buildings were assembled from numbered sheets of corrugated iron.

Galvanised iron, protected by a zinc coating, was a strong and long lasting material that didn't need painting for protection, but many people painted it to improve its appearance. For nearly a century, regular painting was the only way to keep the iron looking its colourful best.

In the 1950s, things began to change. The Chicago based Lithostrip Corporation and Pre Finish Metals discovered a way to successfully bond paint to a galvanized base, and John Lysaght brought the new technology to Australia.

Much time and effort was invested in getting the process and the resulting product exactly right and in 1966 the first coil of COLORBOND® steel rolled off the Number 1 coil painting line at Port Kembla, Wollongong. The new line was the talk of the steel industry, and its new product soon became a firm favourite with the building industry.

Since then, additional paint lines have been set up at Port Kembla, and also interstate at Western Port, Victoria, and Acacia Ridge, Queensland. The latest addition is a new A$150 million line in Sydney's Erskine Park, designed to produce 120,000 tonnes of COLORBOND® steel per year from 2007, enough for 80,000 roofs.