着 is tricky:
"爷爷在穿一件灰色(的)毛衣。" '在穿‘means "Grandfather is putting on‘. I don't think you would say that to mean 'is wearing'.
穿着 means 'I am wearing' or 'I wore' or 'I was wearing', depending on the context. The time clues are in the context, not really in 着。
我（现在）穿着一件红色的外套。(Now) I am wearing...
我（昨天）穿着一件红色的外套。 (Yesterday) I wore / I was wearing