Hi Mike.Yellowing of (Cactus)plants can have several causes.Sudden exposure to strong sunshine if the plant was previously in a shady spot.Mineral deficiency due to exhausted soil.Alcaline water (too much lime).
If I were you I should place the cactus in a slightly less sunny spot and soak the pot in a bucket of (soft) water to wich you should add cactus fertiliser (geranium fertiliser works too).Your plant should plump up after a couple of days and take on a darker green colour.
PS Make sure your plant is well rooted;not freshly potted-up cuttings. Good luck. T.