Burger King's secret fried chicken 'color guide' has been discovered

Burger King apparently uses a "color guide" to help employees determine which chicken fillets are safe to serve to customers. 

Helen Rosner, the executive editor of Eater, tweeted Monday that she found a card titled "crispy chicken fillet color guide" at a rest-stop Burger King. 

The guide displays five different chicken fillets in a range of shades from black to light brown.

Under the lightest and darkest fillets, a message reads "do not use." The three fillets in the middle are "ok," according to the card. 

Burger King did not immediately respond to a request for comment.