Grant fired the home side in front after 38 minutes with his sixth goal in the last five games when he was credited with the final touch as Matt Heath made a hash of trying to clear a corner.
But five minutes later Grant was sent off for a second bookable offence by referee Nigel Miller, who showed eight yellow cards and one red.
Grant's first booking had been for diving, with his second for needlessly confronting Magnus Okuonghae after a tackle.
The Iron had started brightly with winger Andy Barcham twice going close.
First he saw a shot crash back off a post, and then a fierce drive was turned over the top by keeper Mark Cousins.
Colchester equalised in the 74th minute when substitute Steve Gillespie capitalised on a mistake in the home defence to steer his shot past Josh Lillis.
Scunthorpe nearly snatched a winner in the closing minutes but Barcham was denied by a brilliant save from Cousins.
Source: DSG