How does Trypan Blue work?
Trypan Blue is used to selectively color dead cells.
Since live cells still have intact membranes, which are highly selective on what's allowed to pass through, Trypan Blue can only penetrate the membranes of dead cells (since upon death, they become disrupted, leaky, and penetrable). Upon traversing a dead cell's membrane, Trypan Blue simply stains the cell it's signature blue color, and thus shows up as a dark blue in cultures. Thus, any cell that does not appear dark blue (i.e. just ambient-colored blue) is in fact, still a live cell.