A rainbow cactus needs to be set directly in front of a south- or west-facing window where it will be exposed to at least four to six hours of direct sunlight. If it cannot be placed in direct sunlight, set the cactus in the brightest location possible and set up a fluorescent grow light 6 to 12 inches above it. The light fixture should contain two to three 40-watt cool white fluorescent light bulbs and one special fluorescent grow light. Turn the light on early in the morning and leave it on for 12 to 14 hours. Ideal room temperature for this cactus is between 65 and 85 degrees Fahrenheit during the spring and summer and between 45 and 55 degrees Fahrenheit during the winter. It will usually stay cool enough if it is placed on a window ledge.