The yeast strain in this experiment is a mutant that has a breakdown in its unwrapping procedure. It begins to unwrap the adenine, but one component in the assembly process is broken and it drops the product. The DNA, not being able to understand there is a breakdown, still needs adenine and it continues telling the assembly line to make more. The cell mechanisms continue to pull out the package but drop it. This partially unwrapped package builds up and, interestingly enough, this unwanted product turns the yeast cell red.
Some genetic diseases are like that.
The cell works just fine until it runs into a problem of not having enough of a nutrient that it needs or it is accumulating too much of an unwanted product. Then a crisis occurs. But sometimes, the problems caused by a genetic mutation can not be changed, and the effects can be disastrous. In Class Handle the plates as little as possible to prevent contamination. The same medium that keeps the yeast happy and growing keeps bacteria growing.