Why did my colored doughnut change colors?

Some time ago, I made a doughnut recipe, I only made yeast doughnut once a few years ago, I don't have experience with yeasted bakery products. I baked the doughnuts instead of frying. I also added some food coloring I made from beetroot and the color was light pink. The coloring was made by cooking beetroot juice with sugar and adding some lemon juice (if I remember right, it turned out a bit too candy like and thick, so I used more lemon juice and water to thin it), the doughnuts turned out light pink. If I remember right, there weren't any problems when they came out of the oven, some time after they were made, they started to loose color unevenly, some of them lost it much earlier and also in different places, they looked like they weren't colored at all. And I found the amount of flour in the recipe to be no were near sufficient, I kneaded and kneaded but it didn't work so I had to use much more flour (probably double the amount in the recipe), but the food coloring couldn't have added that much water. I suspect yeast might have done it, it also tasted a bit too yeasty so that might have accelerated it. What do you think caused it and how can I prevent it from happening the next time?