Another bare faced pig ignorant lie.

I'm planning an extension to my house, and want to make flat roof
that will last a long time, is maintenance-free and tough enough for
people to walk around on, and is fairly light in colour on the surface.

Can anyone recommend cost-effective materials?

Metal decking.

I'd something rigid rather than joists and plywood with a waterproof
covering, which, in my experience tends to have limited durability.

Metal decking lasts much longer.