Heat is the biggest drawback. Second, fading. Dark colors will fade alot more and change shades more than a lighter color ie. your black roof will look like a dark grey roof in a few years.
As others said, heat is a drawback & it's huge; gonna make the whole house warmer. Nothing looks better than a new black roof though. That said, I went with white when I re-roofed my house just to save some $$$ on electric bills.
you want to look at reflectivity index. black will be low. stick with lighter or plain. lighter colors seem to hold up well. A metal cool roof qualified for the energy tax break a year or so ago when i did mine, the ones that met the criteria were all plain or light colors.
The guy down the street shingled his home in black shingles, he redid it in a very light brown mixed shingle 2 years later, his house became an oven, couldn't cool it.
I've got a black metal roof on my shop and I love it!
It does get hot as do most other colors but white. You dont say where you live, but if your shop is well insulated it's not a problem
Depending on what type of metal roof you are going with I might look at a radiant barrier on the sheeting and then put furring srips on top of that to reduce the radiant heat transfer.
Summer heat may be a problem.
But with correct venting it should be OK.
If you are in snow country the slippery surface will let the snow come sliding off in big sheets.
They make little “fingers” that stick up to hold the snow back, but some people don’t like the look.
May be just make sure you have porch roofs at key spots.
I use grey on all of mine,except the horse barn which is a light tan. White really shows the dirt after a couple years,and as others have said,black will fade and flatten out. The grey does niether.