A while back, I heard that the original distributer cap was grey.  Going off of that and a poor quality picture on BAT, I was able to find a grey YEC branded cap, but didn't know if it was close to the correct color.

Today, I found a picture of an original Hitachi distributor cap:

Putting that here for reference for others.  

 

I personally have not seen a green cap. Interesting to know.

Your cap looks close in color to the 310K BaT 240Z pictured below

https://bringatrailer.com/listing/1971-datsun-240z-124/