This looks healthy to me. Rot would be soft, wet mushy areas, usually at the base. Most cactus will grow pale green in low light, and darker often reddish green in brighter light, which is what you want. Too much red is a sign of stress in light that is too intense. Yours looks fine.

Repotting it out of that wet heavy soil is a good idea. Be sure to use something gritty and fast draining, such as half bagged cactus mix and half perlite. Washing the old soil off the roots will give you a chance to check for rot below the soil line. There is usually brown 'corking' of the skin in this area, but as long as there are no soft mushy spots, it is fine. Good luck!

Jules

I agree with much more grittier potting mix. Wanted to point out that soil seems to be kind of piled up around stems - could be just photo, but it would be better for the plant not t be buried so much. In this case, it would be good to use some gravel-rock topdressing - there is a discussion about topdressing going on right now on this thread

Handling a plant like that can be risky and painful. How I do it is to wrap the plant in dry kitchen paper, then fold a sheet of newspaper over and over, until you have a long strip.

Wrap this strip around the plant, and hold the two ends together, close to the plant, like a collar, so that you can lift and handle the plant fairly easily, without hurting yourself. Don't squeeze too hard, or you may squash the columns....

Had the same problem with my daughters cactus. we replanted in a bigger pot and it is beautiful now