Wow, I'm impressed with your cactus! That's truly remarkable that you've kept it thriving for so long.
While most of the cactus looks healthy, I'm inclined to think the paleness is a combination of lack of nutrients and lack of sunlight. It's really hard for most succulents to get the light they need indoors, and cactus are no exception.
I'd recommend giving it a dose of fertilizer and look into repotting it in new soil. If it hasn't been repotted in a few years, it's likely that it's just needs some freshening up.
You may also find that it will do better with more sunlight. You don't want to take it right in to full sun, but if there's an area of the house that gets more light throughout the day, that would be a good move. It may also be helpful to get a grow light.
Taking it outdoors for a little while can help as well. Start in an area with bright shade all day and gradually move it into an area with more sunlight.