It depends on what type of cactus it is.
"If you purchase desert cacti, you need to make sure they are planted in well-drained compost. They should be well-watered with tepid water in spring and summer. However, the compost should be allowed to almost completely dry in winter months. Especially if they are in cool conditions. This allows the cacti to grow dormant.
During periods of active growth, cacti should be fed about once every three weeks. You can use well-diluted tomato fertilizer for this purpose. Also, desert cacti like temperatures of 50-55 °F. in the winter. You only need to repot desert cacti when the roots completely fill the pot.
Forest cacti are very different. They usually have beautiful hanging flowers that grow from the tips of segmented stems. These stems look like chains of fleshy leaves. They grow this way because they’ve been trained to grow over trees. They are used to shade but they do need some bright light. They need lime-free, light compost that is well drained and should be misted with tepid, soft water. They can be rested in 50-55 °F. Water them moderately and feed them weekly with a weak fertilizer after winter and place them in a room with higher temperatures."
Read more: http://funadvice.com/r/bhjjv2prl6o
tests show that plants seem to flourish under green light. Another little trick, you can water them with sprite every so often, it can really help green a plant up believe it or not(then again, I know it works on flowers & bamboo thus far, don't know how it will do with cactus. Just thought I might throw that out there if you're willing to give it a shot).