I have had to bring mine in from the sun on many occasions - clearly they have no sense, no feeling ..... the dark coated of mine felt really hot to the touch compared to the lemon/white one or the more open tricolours and for sure, I'd not let them lie out there for very long at all before bringing them in.
I've seen two cases of heat stroke, both in dark coated hounds (not mine) and believe me, it's not a good thing to see. One, who took off when my neighbour refused to alter her exercise time to fit the weather, and was later found collapsed in a ditch by a passing rider. She got him home but despite both our efforts to cool him down, he went into a permanent fit and couldn't be saved, even by his vet who we took him to fast. He was a young dog too. So sad.
I'd not just leave mine outside in the full sun on a hot day assuming they'll move into shade or come in.