A few questions come to mind: How old are your boxwoods? Is this April the first time you've noticed this leaf coloration? Have you closely inspected your boxwoods for insects? Or looked for any cankers (black "sores") on the limbs?

Your foliage looks otherwise healthy; I don't see any indications of a disease. Winter cold and wind can cause leaves to turn reddish. A few cultural considerations:

*Boxwoods should only be planted in well-drained soils. Never plant them near downspouts or in any area that stays wet.
*They are extremely shallow-rooted plants and should not be planted too deep.
*The top 1/8 of the root ball ought to be above the existing soil level. Deep planting will usually cause loss of plant vigor and sometimes plant death.
*In early spring, broadcast fertilizer around the base of the plant, just beyond the drip line. Fertilizer particles that come into direct
contact with the roots of unmulched boxwoods can cause root burn. If the fertilizer is over-applied, this will cause the leaves to turn brown and may result in dead branches.
*Established boxwoods should be watered thoroughly at intervals if rainfall is less than an inch per week. Boxwoods lose water through their leaves during winter too.

If you are concerned about this phenomenon, one option is to send pictures and full descriptions to the MSU Plant and Pest Diagnostics lab on campus. Please email digital images priorto bringing or sending physical samples. Images can be sent to <personal data hidden>.

They may be able to ID your problem without seeing a sample. If they do request a sample, the link below tells you how to submit one.

https://www.canr.msu.edu/pestid/submit-samples/

Due to the coronavirus, they may not be currently accepting drop-off samples; they must be mailed. Give them as much information as you can possibly think of, i.e., when did this problem first begin, do you see insects, drooping shoots, cankers (oozing sores) on the trunk of the tree, etc. Mailing samples on a Monday is the best time to do this.