Final answer:
Gram-positive bacteria appear purple after the first dye is added in a Gram stain due to their thick peptidoglycan cell wall which retains the crystal violet dye.
Step-by-step explanation:
When you are performing a Gram stain on gram-positive bacteria and stop after the addition of the first dye, which is crystal violet, the appearance of the bacteria is purple. This is because gram-positive bacteria have a thick peptidoglycan layer in their cell wall that retains the crystal violet dye, making the cells appear purple.
In contrast, if a gram stain were performed on an L-form bacterium, which lacks a cell wall, the bacterium would be colorless because it does not have the cell wall structures necessary to retain the crystal violet stain.