If it is methylene blue, methylene blue is a non-toxic basic dye, which is blue when oxidized and colorless after reduction. Living yeast cells have strong reducing ability and can reduce methylene blue, so they are colorless and blue when they die.
In this way, if the concentration is high, it is very likely that yeast, especially newborn and aging yeast, will be dyed light blue because of its weak reducing power, so the number of dead cells will increase.
If the time is too long, the number of dead cells will be too large, because yeast cells will gradually die without the best culture conditions.
To sum up, if the concentration is too high and the action time is too long, a large number of cells will die.