what kind of soil are they in? what kind of pot - glazed/unglazed or plastic? does the pot have drainage holes in it? what kind of light is it getting? is it inside or outside?

Sorry, Both are in a soil mix I make of about 3 parts commercial Cactus potting soil with about 4-5 parts coarse gravel and about 2 parts sand.
Terra-cotta pots, One glazed one unglazed. Both with a Drainage hole.
Both outside in partial/full direct sun. They're against a building where they're in full sun from around 10am until about 3pm.
They were doing fine until the start of this winter when they both started yellowing. Both have new growth too and it's also yellow and doesn't look very strong or healthy.

"Peruvian Apple" often refers to Cereus repandus, a tall, eventually tree-like, columnar cactus. It is not very hardy and I suspect this might be the reason yours has gone yellow in winter.

Thanks shrubs_n_bulbs but if it is the same, they seem very hardy here in Florida where they do grow into Huge trees. I'm thinking it's in need of some fertilizer or something that is missing in the soil. I'm going to repot and see what happens.

Well, it's been a while since this post and I lost one of my plants. But... I finally found out what it was that turned them yellow. I started looking REALLY close to the plants... examining them very carefully. I discovered, right under the fine white hairs at the base of each areole, under the smaller spines were scale insects! The little buggers were living under just about every areole! They sucked one completely dry and almost killed the second. As it stands, I cut the top off the survivor (just in case), sprayed it really good every other day and did the same to the bottom half. The top half has turned green again and is growing and the bottom half is still a bit yellow, but is turning greener.
When in doubt... look harder!