Since the roots seem to be in good shape, I am suspicious that this is a nutrition issue. It could be a nitrogen deficiency. I would recommend fertilizing them with a good quality slow release ORGANIC fertilizer such as Espoma Plant-tone according to the label directions. Do not use a chemical fertilizer like 10-10-10 this late in the season because the nutrients in these fertilizers become available right away and may stimulate rapid new growth that won't have time to harden off before freezing temperatures set in. This would make your boxwood susceptible to winter damage. In the late fall you may also want to consider spraying them with an anti-desiccant like Bonide Wilt Stop according to the label directions. This will help protect them from winter burn and winter damage.
Hopefully in the spring your boxwoods will come out nice and green.
The other thing is that it has been very dry the past few months (up until last week!) and if you didn't provide supplemental water for your boxwood, they may have been water stressed. Too much or too little water can cause boxwood foliage to decline and become yellow. They may green up after the rain but it would still be a good idea to fertilize them with Plant-tone.