Cacti desire to photosynthesis through their skin.  To retain water, they open their stomata at night to absorb CO² during their CAM cycle.  That carbon dioxide is stored in cells till daytime.  When the sun is shining, they commit themselves to photosynthesis just like any other plants.  That green chlorophyll is part of the process to absorb sunlight.  When we have variegated plants, we are dealing with a mutation that in actuality is not helpful in the process of photosynthesis because the lack of chlorophyll.  But to the demise of many a yellow plant, the collector in us, likes to cultivate these mutated plants that expose themselves to being green and yellow (or just lack of chlorophyll).  When dealing with variegated plants, we should always know they want to revert to full green or even full yellow.  It’s not a question of if, but when.  As collectors, we just know there are some that can be so well patterned for ever, and others that will fight you every chance they get.  Some find them ugly, while many of us who have collected for years, find their rarity and difficulty appealing to our garden.