Box going a yellow-Orange-red colour is a sign of stress. I live in area of heavy clay and quite windy, so lots of the Buxus round here, including a lot of mine, are copper in colour. I have improved the soil and watered them and have started to see some new growth that is green. My suggestion is that you keep your Box well watered and make sure that the roots are no compacted and give it some feed.

There is Box blight, but that makes the leaves look dead and you can see blackened bits and spores (red in colour). If your Box looks like that then it needs treatment - but coppering, what sounds like what you have written, is a sign of stress.