I'm confused - Mexican Orange is usually the common name for Choisya - as far as I'm aware, there is no variegated version of Choisya available. Assuming your plant is Choisya, then I'll make another assumption - that its 'Sundance' or another yellow leaved form, and you've got green growth on it as well. Trace the green growth back to its point of origin, and clip out the whole branch from there. This also applies if you've a green shoot on a variegated shrub, but I'm intrigued by the idea that you may have a variegated Choisya...

Graham Rice in "Transatlantic Gardener" (transatlanticplantsman.typepad.com):

"Discovered by British nurseryman Peter Catt, it’s a yellow-leaved form of the familiar evergreen shrub Choisya ternata with the same fragrant white flowers. You now see it in front gardens, in particular, all over the Britain. It’s interesting to know how it originated. Peter explains.

"This very popular plant started its life when, in 1978, I spotted a very small leaf, low down on an old Choisya ternata from which I was taking cuttings. I took the cutting on which this small leaf was and rooted it. When it came into growth I removed the shoot from the top, which encouraged the side shoots. These I also removed. This forced a growth from the leaf that I had spotted which was about the size of my small fingernail and had a white edge. I had envisaged a white edged variegation. To my amazement out came a golden shoot and having cleaned it up in a micro-prop lab, the plant was launched at the Chelsea Flower Show in 1986."

So from one leaf the size of a fingernail came one of the most popular of garden shrubs.

But it’s not perfect. It sometimes reverts to plain green and, as can be seen in the picture, the green shoots outgrow the yellow ones so must be removed. It also sometimes bleaches and scorches in full sun, especially on dry soils and in dry summers. And it’s not happy in drying winds.

any varigated shrub or tree that reverts needs what bamboo sais done to it . i had to do it with my acer tree as it would eventualy all change back to its origional colour if not .