爷爷在穿一件灰色(的)毛衣 = grandpa is wearing a gray sweater. 爷爷穿着一件灰色(的)毛衣 = grandpa wore a gray sweater. I hope this answer your question.

一般都是用“穿着”。 of course, if he is really putting on the clothes, you can use"在穿" or “正在穿”。

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'. whereas 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

or you can.say.“爷爷穿着一件灰色（的）毛衣”

不可以 no