The Colts should have been on the phone with Colin Kaepernick the second Andrew Luck retired ... so says Brandon Marshall, who tells TMZ Sports Kap's got more upside than Jacoby Brissett!!!
"I know Jacoby knows the offense, but I think the upside of Kap is better," Marshall says ... "It's higher, you know what I'm saying?"
Kap's been famously out of the NFL since taking a knee during the national anthem in 2016 .