Tomatillo is more sensitive to cool temperatures than tomato. I don't know your location, but some parts of the US are still having cold snaps. Northern Wisconsin just had a frost advisory a few days ago. Hopefully that is the problem, and the plant(s) will green up when temperatures warm.

Tomatillos are very temperamental as transplants. But the funny thing is that if seeds from fallen fruits volunteer the following year (which they do very well) those plants tend to be exceptionally healthy. Last year's crop was the best I never planted (lol). Like ground cherries, if you plant them once, you will have them forever - for good or ill, depending upon whether you want them or not.