You're pretty simple if you conclude I'm referring to the amount of melanin in someone's skin, you lefties are always obsessing about race lol.
It obviously doesn't matter what someone looks like, maybe only to you guys... but what I'm referring to is values. So yeah, needless to say black and white people with similar values should have no problem achieving a common goal.
But when for example you flood a country with western Christian values, with immigrants who believe in sharia law, then diversity is not a strength. When you try to introduce mentally ill people who are confused about being men or women into your army, you are not making it stronger. When you try to fill your fire department with physically weaker females, you are not making them more effective.