The difference between heaven and hell is not that heaven will be ethnically diverse and hell won’t be. Both will be diverse. All races and all ethnicities and all cultures will be present in hell.
The difference between heaven and hell is that in heaven—that is, in the new heaven and new earth—there will be perfect racial and ethnic harmony, but in hell, racial and ethnic animosities will reach their fullest fury and last forever.
Paul spoke of the ethnic diversity of hell in Romans 2:9,
There will be tribulation and distress for every human being who does evil, the Jew first and also the Greek.
“Jew and Greek” was shorthand for all ethnic groups.
We know he is talking about hell here because the opposite is eternal life:
To those who by patience in well-doing seek for glory and honor and immortality, he will give eternal life. (Romans 2:7)
Therefore, everyone who loves racial diversity and racial harmony will call all ethnic groups everywhere to believe on Jesus Christ, the Son of God, because
Whoever believes in the Son has eternal life; whoever does not obey the Son shall not see life, but the wrath of God remains on him. (John 3:36)
Heaven will be all the sweeter because the ethnic diversity that God designed for the good of his creation and the glory of his grace will last forever in perfect harmony and joy.