Gentian violet colouration of gram-positive bacteria is due to -
GRAM STAINING 
The gram stain differentiates bacteria into two broad groups
1. Gram-positive → Resist decolourization and retain primary stain, appear violet.
2. Gram-negative → Decolourised by organic solvents, and therefore, take the counterstain, appearing red. Smear stained with gentian violet (primary stain)
↓ Application of dilute solution of iodine
↓ Decolourisation with an organic solvent, e.g., ethanol
↓ Counterstaining by carbol fuchsin
↓ ↓ ↓ Retain primary stain (violet)
↓ Gram-positive Take colour of counterstain (red)
↓ Gram-negative 
Why do gram-positive organisms retain primary stain, but not gram-negative organisms.

 In gram-positive bacteria, there is a thick layer of peptidoglycan (cell wall) just outside the bacterial cytoplasmic membrane.
 This thick wall traps the primary stain (crystal violet) which is not decolorized by ethanol.
 In gram-negative bacteria this peptidoglycan (cell wall) layer is thin and just outside this peptidoglycan layer is outer cell membrane (which is not present in gram-positive bacteria).
 This outer cell membrane is partially dissolved by ethanol, thus washing out the crystal violet and allowing the counterstain to take
Peptidoglycan
Capsule
Cell membrane
None of the above