[1] Gram-positive bacteria have a thick peptidoglycan layer in their cell wall, which retains the crystal violet during Gram staining, resulting in a purple color.
Gram-negative bacteria have a thin peptidoglycan layer which does not retain the crystal violet, so when safranin is added during the process, they stain red.
The Mycoplasmataceae lack a peptidoglycan layer so do not retain crystal violet or safranin, resulting in no color.
The Chlamydiaceae contain an extremely thin peptidoglycan layer, preventing visible staining.
Peptidoglycans are the site of action of beta-lactam antibiotics such as penicillins and cephalosporins, so mycoplasma are naturally resistant to these drugs, which in this sense also makes them “atypical” in the treatment of their infections.