Intersectionality: the overlapping, interaction, and interrelation of different oppressions.
Derailing: wrenching the conversation away from its original topic to talk about something that, almost always, is more comfortable for the derailer, thereby diminishing the importance of the original topic. For more information, I highly recommend Derailing for Dummies; its tone is more sarcastic than this essay, but it is excellently precise and informative.
I cannot give you any bright-line rule for when pointing out an aspect of intersectionality becomes derailing, because there is none. But, first, you should be aware that people often say “well, what about X?” because, whether they realize it or not, they are uncomfortable talking about racism. Think about whether this applies to you.