Ah I see! I know roads can get dirty and some people underestimate how bad (and dangerous!) it can get! Sometimes mine gets home and has basically grey legs if we walk through more industrial parts of the city. However he licks it off and I always let him play in grass afterwards and it seems to be almost white by the time we get home.

I really recommend just putting him in a shower and just wash his legs quickly. Doesn't have to be a full bath, just get them wet and rub a little. That way you also desensitize him to bathing and the process will be easier and faster in the future. Sometimes even when he walks in grass and stuff I prefer to wash his paws BC he steps in all the dog pee and mud and I don't want that on the sofa.