Definition of Dye- Reduction Test?
Dye reduction tests (and there seem to be loads of different ones) are simply assays in which a dye becomes decolourised to give you a visual indication of whether a certain process is occuring. Here you can find an example of a dye reduction test with Methylene Blue and Reazurin which indirectly measures the bacterial densities in milk and cream. See this google book for more info.