What is red and bad for your teeth?

A brick.

Red is a colour. Bad for your teeth means something that is detrimental to your dental health.

Magma

A cherry Jolly Rancher candy

Probably Lightning McQueen, biting into him probably wouldn't be good for your teeth

A brick

Evil red tooth melting acid 😈

knuckles the echidna

A ginger dentist with paranoid schizophrenia who's convinced that Zombie-Aliens have sent you to spy on them, so they sedate you and violently drill away at your teeth to find the hidden microphone

The red M&M

Mountain Dew: Code Red

The book Quotations of Mao Tse-Tung.

It can't be good for your teeth.

Betel nut chewing

A bleeding hand punching you in the face.

Fire hydrant

A newspaper?