Can wild yeast/bacteria change the color of the wort?

This is my 3rd all grain brew. Maybe my 10th or so batch overall. I made a 6 gallon batch of single malt (2row) with Willamette hops. (The single malt is more of an experiment.) Without going into times and schedules, in the end I was left with 6 gallons of light golden amber wort. Split the batch into two 3 gallons batches. One in a glass carboy, the other in a 5 gallon bucket.

I made a batch of Nottingham yeast from a dry packet in a 1/2 cup water. I (think!!) that I added it to both batches. The next morning, crazy bubbling from both batches.

1 month later I add lots of pureed peaches and strawberries (saoked in vodka for a week) to glass carboy since the bucket is a pain in the butt to open. One month after that, I add isinglass to both because the fruit is not settling and I just added to the bucket just because.

2 weeks later, I go to secondary. The fruit in the glass carboy looks ok. Cloudy, but still that golden amber. BUT, the wort in the bucket is dark brown. Coffee colored!! WTH?! I'm pretty anal about cleanliness. But, is it possible for wild yeast or bacteria to change the color that much? It smells like beer, maybe a little sour but nothing that I didn't try it and say, "Still tastes like beer..." Anyone?

Sounds to me like your batch without the fruit just got really clear as everything dropped out, which will appear as a darkening. Have you checked the color in a sample tube or glass? Bet it's crystal clear and lighter.

As was mentioned, once the yeast drops out the beer looks a lot darker. Take out a sample and put it in a glass and see if it's really dark or just appears that way in the carboy.

The only thing I know of that can cause noticeable darkening in beer is oxidation. Oxidation can make a beer get much darker, "browner" looking.

It was definitely the yeast being in suspension that made it look lighter. I just checked both worts and they are now the same color. The fruit wort is still a little cloudy, but is now closer in color to the other.

Cheers!