I can’t tell if you’re serious. It’s been like that forever. Black ink on face and green ink on the back.
Some of the earliest Federal currency, in the 1860s and 1870s, used a green "protector" tint on the face side, to thwart photographic counterfeiting. The overlapping black and green printings just looked like a muddled mess to the black-and-white cameras of the day. But the ink used for the green tint had a tendency to show through on the backs of the notes. To hide it a bit, the backs were printed in green ink. That started the tradition of green backs that's still going strong today. Though there have been a few exceptions over the years -- Brownback NBNs, yellow-back gold certificates, and so forth.
