My Golden Barrel Cactus has started to turn Yellow

It has been lightly watered?

These need very good light when mature. In the wild, they often prefer shade as seedlings under a bush etc and can take more and more sun as they grow and get their roots down. They prefer airy conditions and I would suggest that next summer, you give it a spell outside. Like virtually all true cacti, when watered, they should be given a good soaking so that ALL the rootball gets moist. In fact, in active growth in summer, they will take the same amount of water as most pot plants only letting them dry off a bit between. Watering should be reduced in autumn with several weeks between watering in winter indeed a month to six weeks is fine. Give it a liquid feed occasionally in summer too.

ps You will have to bite the bullet and re pot it sometime as they do need to grow. It will be interesting to see what the roots are like here too when you take it out. If water hasn't been getting to the ones at the bottom, or indeed it has been constantly kept too moist, there may be either dead roots or rotten roots.

The trick for repotting cacti is to wrap the plant in news paper tied on with string.