Brandon Marshall Says Colts Should Sign Kaepernick, More Upside Than Brissett

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 ... but his ex-college teammate thinks now is the PERFECT time for a squad to bring him back in.

"Honestly, man, a team like the Colts, they could've used him," says Marshall, who played with Kaepernick at Nevada from 2008-2010. "The Dolphins, they could've used him."

"He could upgrade that roster immediately," Marshall added. "Even with being out of the league for three years, he could definitely upgrade that roster. I think that's an easy pick."

Seems Colin would be more than ready if one of those team's GMs called ... he's posted a couple workout videos this summer -- and he sure looked good throwing to Odell Beckham Jr.

But, even if a team doesn't come calling this season ... Marshall says he's damn proud of his friend, telling us, "God chose him for a reason to be the face of the movement at the expense of his career, unfortunately."