Answer:
In acid-fast staining, carbon fuchsin is used as a primary stain which dissolves the mycolic acid present in the cell wall of Mycobacterium smegmatis and penetrates through it which results in staining Mycobacterium red.
Staphylococcus aureus cell wall does not contain mycolic acid so carbon fuchsin does not penetrate its cell wall, therefore, it becomes colorless after destaining with acid alcohol.
After destaining step methylene blue is added to stain non-acid-fast bacteria blue. So if I mistakenly forgot to use methylene blue during the procedure Mycobacterium smegmatis will appear red due to carbon fuchsin present in their cell wall and Staphylococcus aureus will appear colorless because it is destained.