if the pH is low (below well of 7.0) in your culture, its colour starts to shift to yellow (means something mixed by blue and yellow, likely greenish). but this is not the redox change due to viability being detectable as reddish colour.

It depends on the number of cells and cell type. 

You may consider optimising the time taken to reach plateau using different number of cells before commencing the actual experiment.

Yes, in most bacterial cultures, you can see the change in color in a few minutes.