It is when the color of ones face turns red/pink when they are caught guilty in commiting the `acta-farter`
After George Farts:
Peter : aaaaahhh god...Who was that..??
George (looking innocently at Anna) : ohhh God Anna that one was Bad.!!
Anna : George you bloody acta-farter...!!!
George : uhh no im not an acta-farter
Peter : hahahahah look Anna George has a stinkypinkyfartface