When living yeast cells were placed in congo red dye and examined under the microscope, the yeast cells remained colorless. however, when placed in methylene blue, they became blue. later, dead yeast cells were placed in congo red dye. these cells turned red. explain these three observations.

Well, to be honest, some of that doesn't make sense. We'll take each observation individually. 

1. Living yeast cells were placed in congo red dye, but they remained colorless. 

This makes sense. Congo red freely diffuses from high to low concentration across the cell membrane, without using any energy. The cell then pumps the molecules back out of the cells, moving them from a low concentration inside the cell to a high concentration outside the cells. This causes living yeast cells to remain colorless in congo red dye. 

2. Living yeast cells were placed in methylene blue, and they turned blue. 

This is the one that doesn't make sense. Yeast cells that are living contain an enzyme that decolorises methylene blue, whereas dead cells do not. When cells from a yeast sample are suspended in the dye, it penetrates into all the cells, but is broken down only by the living cells. So you can distinguish between living and dead cells by examining them microscopically: dead cells are stained blue and living cells are unstained. 

3. Dead yeast cells were placed in congo red dye, and they turned red. This one also makes sense. As I said in #1, living yeast cells actively pump congo red back out of themselves. If the cells are dead, this pumping does not occur, and the congo red stains the cells. The same would be true if you placed dead cells in methylene blue. The enzyme that normally breaks down the methylene blue would not be working because the cell would be dead, and the cell would turn blue. I hope i helped :p