A new teacher asked a child to go inside for rug time and the child said, "You can't tell me what to do, you're black." ?

I would say "I am the teacher and you have to do what I tell you to do." 

Trolling.  This didn't happen.

"Yes, I am black. I am a black teacher. Now go inside."

Don't get angry, don't shout, don't try to push or pull the child into the building. Just glance calmly around to see what other children would have heard this, and when you have the opportunity, write their names down so you don't forget.

There is no point sending a child young enough for "rug time" to the head teacher. After school go and report the incident to the head teacher, who will certainly then want to call the parents in and have a conversation with them.

When asked for your account of the incident, stick strictly to the truth, and don't exaggerate or put words or thoughts into anyone's mouth.

Yeah, this didn't happen. 

"It makes no diffrernce what color I am, I am the adult, I make the rules."

Sent to the office for racism 

I would send the child to see the headmaster

and then arrange to have a serious talk to the child's parents

Wow. You can say "I might be Black but I'm still your teacher!! Now go inside for rug time...Please!!"