Green displays are usually VFD (vacuum fluorescent display) and these are totally different to LED's. They are a glass envelope like a valve and they also have small heaters (filaments) like a valve. Old VCR's and the like usually used these.
https://en.wikipedia.org/wiki/Vacuum...escent_display
Having said all of that... you can get green and yellow LED 7 segment displays but I've never seen them used in clock radios.