Can someone who is color blind still join the military?

Color blind can join: Apparently all army enlistees are tested for color blindness, but being color blind will not disqualify you from service in regular enlistment. However, certain military jobs like airborne units and special forces may require normal color vision. The tests used are the pseudoisochromatic plates (pip) or farnsworth lantern (falant). In some cases vivid red and green lights must be distinguished.

That Depends.: "failure to pass a color vision test is not an automatic disqualification. Although there is no standard, color vision will be tested because adequate color vision is a prerequisite for entry into many military specialties." see dod directive 6130.3 & dod instruction 6130.4.

Yes: Color deficiency is not tested during entrance physicals and color normality is not a criteria for most military duty. There are a few tasks in the military in which color recognition is required, so you would not be able to do those but that would be told to you after you join.