It's a little disgusting that there are really people out there who think POC can be racist.
But before you write me off as another bitter "SJW", understand that I couldn't be racist even if I tried. I don't have the power to. White people will use racism as a means of systematic oppression of PoC. A PoC, however, is not capable of this, and therefore cannot be racist.
To tackle the point that "they can be racist towards each other", this is a case of what is called "colorism," which is inextricably tied to internalized racism and self-hatred. It can be common in ethnic communities - but all of that is nothing more than just another harmful side effect of the main issue, which is, of course, racism.
It is a system that continually harms, endangers, and discriminates against people of color and mainly only white people benefit from it.
Think of racism as the core, or starting point, of all of the harmful stereotypes, discrimination, and hostile prejudices targeted specifically at race. Of course, people of color can partake in this "core," but it will likely not advance towards any of the above mentioned topics. When a white person participates in this "core," however, it can easily disperse. This is simply because the system that our society has in place perpetuates the spreading of that core specifically when it is directed against people of color; this leads to discrimination, harmful stereotypes, hostile prejudices, and much more as mentioned earlier. Typically, we don't see these kinds of "side-effects" happening to whites because, as I said, our system does not usually allow for it.
Finally, it is the spreading of this "core" that encompasses the definition of racism, not just the core itself. A whole is nothing without its respective parts.
Is that too hard to understand?