FWIW, I’ve found that fresh beets result jn a different colour than beet juice, and there are also different types of beets, some which have a lot less colour. I’d also think there might well be beet juices out there with red colouring in. And then the colour might depend on what else you ate, how much fluid you took in, etc. All of that seems more likely to me to cause variation than a change in iron levels.