color change in grape juice

It has to do with the anthocyanins in the skin. Anthocyanins are phenolic copmpoounds like tannins. Their color is dependent on the pH they are at (as well as other things). At an acidic pH they appear red, turn colorless from around 4-6 and then turn green at basic pH.

So if your wine is turning color during fermentation, you are either raising the pH too high (>4, which is bad) or the pH is decreasing (which is not so bad).

There are a lot of different anthocyanins though, and the exact pH at which they change is different for each. I notice that my rhubarb wine changes color from green to red during fermentation. I am assuming that is becasue the yeast is actually lowering the pH.

I'd say it's going from purple to red.

Still hard to say. The anthocyanins could be moving to colorless giving a lighter color, or the green color could be movie toward colorless making it appear more red. The only way to know for sure it to take a starting pH and an ending pH.

There are other possibilities, these color compounds can oxidize/reduce also. In freshly fermnted wine, full of sulfite there is very low O2, the wine will appear redder and more vibrant. As the wine ages, some of these color compounds oxidize and turn less ruddy, more rusty in color.

If you want to go out and make 100 grand a year, become a food color scientist or a food flavor scientist. It is all about how to keep your wine red and your bread fresh so it can sit on the shelf for weeks, months or years.

I'm not sure this is what you are looking for but the end of fermentation is always visible by a color change. It goes from murky to the color of the wine and stratifies. Clearer wine is a deeper version of the the femrenting wine color. Joe