What is happening in the food vacuoles that causes the yeast to turn blue?

Answer;
Food vacuoles that causes the yeast to turn blue indicates active digestive processes in the cell.The blue represents the mature food vacuoles as some yeast cells have now entered, and beginning the digestive process. 

Explanation; 
The ingested yeast cells appear red during early stages of being eaten. Later some of the yeast cells will enter into the food vacuoles to which acidosomes have fused, lowering the pH. The yeast cells within the food vacuole will turn blue, indicating active digestive processes in the cell. It is used for yeast uptake in the paramecium food vacuoles.