10:37 AM PT -- Harrison just issued a statement on Twitter saying, "I apologize to anyone that I offended, wasn't meant to be hurtful to anyone. God bless."
However, we spoke with him on the phone and he didn't back down -- essentially saying the comments he made on the radio show were his real feelings and he stood by them. Colin Kaepernick is not a TRUE black man ... because he's mixed race ... so says ex-NFL star Rodney Harrison.
The former Patriots safety went on SportsTalk 790 in Houston -- and not only blasted Kaepernick for his National Anthem protest, but says he doesn't REALLY understand the plight of black Americans because his birth mother is white.
"I'll tell you this," Harrison said ... "I'm a black man and Colin Kaepernick ... he's NOT BLACK."
"He cannot understand what I face and what other black men and black people face on a every single [day] basis."
Check out the audio ... Harrison essentially says there's a scale that measures blackness ... and Kaep doesn't measure up.