The culture might not be stained properly or the culture might be too old.
Explanation:
Gram staining is done to differentiate between gram-positive and gram-negative bacteria according to the difference in their cell wall. In gram staining, the gram-positive bacteria appears violet and gram-negative bacteria appear pink in color.
Therefore if a pure culture contains have both red and purple cocci that it can be concluded that the gram stating was not done properly like too much destaining was done or too much safranin was used or maybe the culture is too old so that cell wall of some bacteria degrades.
Assuming purple is dominant, the genotypes of the parents must be Pp and pp, such that half the progeny have the dominant allele (P_) and half have the recessive phenotype (pp) 25% possibility of white-flowered plant life and 75% possibility of pink ones
