Ah this is not good. Sounds like a case of reversion to me. This sometimes happens sporadically throughout the life of many variegated species, Japanese maples being a bit worse than some shrubs in my experience.

Over fertilization, stress, and apparently a bad cold snap (I hadn't thought about this being a problem but it makes sense) can cause this. If I were you I would wait a bit longer to see if any of the leaves have the normal variegated growth. The reverted growth will likely need to be pruned off, however you might want to wait until next year to see if it goes back to the variegated form next spring. For a large tree I would reccomend hacking off that reversion, but since yours is still small there may be nothing left if this is done now.

Orido Nishiki has variegation but it sometimes is sparce...I don't know about Matts reversion comment in my short experience with this plant you'd be cutting about 1/3 or more off each year if that was the case ...The freeze messed up mine pretty bad too but has shown quite a bit of new variegation maybe more than last year ... I also have a Nishiki gasami and it has NEVER been more variegated it is absolutly stunning!!!...I think stress tends to cause more variegation not less. But on trees that do revert like koto ito kamachi and beni komachi I have seen a definite problem being the original hardier plant was virtually untouched but the newer specified cultivar was damaged and has taken a back seat...how I will remedy this is unknown at this time. As i said I don't think the Orido n. is in that same class.I do agree with Matt on one point ...be patient and wait. David

I was thinking 'orido nishiki' was as variegated or more than say 'butterfly'. I do not own this cultivar and have only seen pictures which lead me to believe it was quite variegated.

I have seen where environmental stress has caused reversion on a few 'butterfly' trees. Scorched leaves on vigorous twigs sent out reverted growth during second summer growing period. However, this is the opposite environmental extreme and I only make the assumption that the cold defoliation may have caused some reversion.

Regardless, still wait.

The variegations on many varieties will disappear when the tree is fertilized to heavily or simply planted in fertile soil. Many times the variegations reappear when the tree has used up the nitrogen that is pushing the photosyntheses.

I never fertilized the plant at all since I had read it's not a good idea. I prepared the planting soil @ 1/3 part of pine bark "fines" soil conditioner, some white granules (lava rock? I forget) that had been recommended. Those were mixed with native soil which has NEVER been very fertile, HaHa.
Thanx for all the comments. I'll wait maybe another year, then replace it.
So many fabulous J. maples, so little time!!