Why did my coffee change color?

I made a pot (with my electric perk) last night and transferred it to a thermal carafe as I always do. I then forgot to empty the carafe last night (I drank about half of it). To my surprise the coffee had changed overnight. It lost the warm brown look and changed to a cooler brown shade. It also had a really weird, flat flavor (I had a sip). I have forgotten the coffee overnight in the carafe before (It is like a thermos and seals keeping the coffee warm for a long time) but have never seen change like this. It was still warm and in the carafe maybe twelve hours overnight. Any ideas as to why it did this? It was store blend (I was low on cash when I got it so could not go for the highest quality). The picture below shows what I am talking about. The coffee on the left is the coffee that changed overnight. The coffee on the right is from a fresh pot.