The line can be drawn when you might disfavour any one race or make it biased towards another race.
For example, in trying to be diverse, when you instead either:
* portray White people or any other race negatively (as incompetent or mostly as villains, or frequently condemning them),
* or treat one race as "better" than Whites or other races, as if they are perfect or cannot do wrong,
you might get racist.
I suggest two things:
* Antagonists, White and non-White alike, should be portrayed objectively. They can be as evil as they can, but race is not a factor. The same likewise for protagonists.
* If you want diversity, try to make it relevant to the setting, with as much research as you can: settings such as medieval Bohemia and Japan do not need too much diversity, while it is more tolerable for 20th century New York, Samarkand, and science fiction. An all-White setting such as Episode IV in Star Wars can be acceptable, nonetheless, because you may assume it to be European.
Try to watch some racist films (e.g. stereotypes), and think what would it be like if another race were portrayed like how some were shown in the racist films.
While stereotypes may tell some things about some races, they are unnecessary as people within races can be individuals, some of which who might disagree with an abusive or aggressive majority.