Jim Brown says he WANTS to support Colin Kaepernick's cause -- but admits he would NEVER disrespect the national anthem by taking a knee during a game. 

Brown opened up about Kaep to The Post Game ... saying, "I want to be in his corner, and I do think, 'God bless him' -- but noting, "Colin has to make up his mind whether he's truly an activist or he's a football player."

"Football is commercial. You have owners. You have fans. And you want to honor that if you're making that kind of money."

"I can't be two things at once that contradict each other. If I sign for money, then the people I sign with, they have rules and regulations."

As for the protest, Brown says ... "I'm going to give you the real deal: I'm an American. I don't desecrate my flag and my national anthem."

"I'm not gonna do anything against the flag and national anthem. I'm going to work within those situations. But this is my country, and I'll work out the problems, but I'll do it in an intelligent manner."