Trypan blue is a vital stain used to selectively colour dead tissues or cells blue. It is a diazo dye.
Since cells are very selective in the compounds that pass through the membrane, in a viable cell, trypan blue is not absorbed.

However, it traverses the membrane in a dead cell. Hence, dead cells are shown as a distinctive blue colour through a microscope.
Since live cells are excluded from staining, this staining method is also described as a dye exclusion method.