The two billboards display black-and-white images of three American Indians with the caption, "Turn in your arms. The government will take care of you."
While the history of American Indians has been used in debates concerning gun control, the photo used on the billboard has sparked outrage, KUSA-TV, Denver, reported Monday.
"I think it is insensitive, because even though it is what may have happened in the past, people are still living that. Relatives are still living that," said Kerri Salazar, a Greeley, Colo., resident and American Indian.
"I was disgusted. I was disgusted," Greeley resident Maureen Brucker said.
KUSA said it contacted the advertising agency that owns the billboard to try to speak to the individuals who paid for the message, but was told the buyers wanted to remain anonymous.
"You don't do this. You don't take something that is indicative of an ethnicity and use it to prove your point. It is just not acceptable. No matter what the ethnicity, no matter what the point, you don't do it," Brucker said.
Salazar and others have asked that the photo be removed and not included in the gun debate, KUSA reported.
"My hope would be is that they can just take the picture down, leave the saying. You know I get it. Believe me, I get it," Salazar said.