Variegation is the term used to describe leaves with different coloured markings, for example in the Euonymus fortunei pictured below.
Variegated plants are created from propagating totally green plants which have developed a genetic mutation, resulting in different colour zones appearing in the leaves. It is rare to find variegated plants occurring naturally.
What is reversion?
Reversion is when some of the variegated stems start producing leaves that are totally green, that is, the plant is starting to revert back to its original green origins.
Reversion is a problem because the green reverted leaves contain more chlorophyll than the variegated leaves, and chlorophyll is the pigment used by the plant to harvest energy from the sun. This makes the green reverted leaves, more vigorous than the ornamental variegated leaves, and eventually your plant will become completely green (and boring) again.
What to do
The only course of action is to prune out all reverted growth, either by removing the whole stem, or by cutting back to wood that contains variegated foliage