So much wrong here. Asphalt shingles neither "retain heat" nor "warm themselves." You really do not want either property in a roof covering in a climate where it snows because that will cause the formation of ice dams. If you have snow on your roof, it obviously covers the black shingles - now you have a white roof anyway. If it is winter and you do not have snow on your roof, it is still not worth it - solar irradiance at 50° latitude (approximately Timmins, Ontario) is going to be about a factor of 7 less in January than in June. You are much better off with a white roof in northern Canada to reduce peak heat stress in the summer, which will continue to get worse and worse due to greenhouse gasses.
We had a steep roof in the Okanagan which rose about 20ft to the peak. That extra room was problematic in winter for heating but a life saver in the hot, sunny summer. The loft was unbearable during the summer.
A neighbor with a low roof would run a sprinkler on his roof all summer long, that's how bad the heat would get.
My previous house already had a light colored metal roof, and my current one is shingles that are not due to be replaced for another ten years or so, but these are good things to think on.
Plus, in practice, you'll want a ton of insulation between the interior space and the roof anyway. So if you've done a good job there it's kind of a moot point wrt/interior comfort. But sending that power back into space rather than absorbing it can reduce thermal cycle stress on the roof and keep the outdoor space nearby cooler.
The other side is that having a roof which warms itself in winter is better for reducing snow accumulation. Though that’s also less of a consideration for a slippery metal roof.