Green mikan?

When visting the japanese countryside I found one stall selling mikan fruits, the original orange ones, but there were green ones too which reminded me of lime. I took a picture of the sign if needed.

Is this non ripe mikan perhaps?

I had to throw them away unfortunately as I went home yesterday but still curious what it was.

Green color mikan(called "ao-mikan"(blue mikan)) tasts sour and sweet, yellow color mikan is a bit sour and mostly sweet.
Green mikan is already riped but still sourness remain stronger than sweetness.
It depends on which mikan you like but there are large number of people love green mikan rather than yellow mikan.
Green mikan is available from September, yellow mikan is available from November.
You should try green mikan one bite before throw them away. You may love them.

I did try to peel them open as with orange mikan but I thought the skin was to hard so I assumed it was a lime. Unfortunately I already throw them away. Hadnt the food restrictions been there I would have bought a lot home. Thats what I love about Japan in autumn, the mikan.

Yes, green mikan has harder skin on, not as soft skin as yellow/orange mikan. You need to peel with a knife if it's harder.
But green mikan contains rich Vitamin C, β-carotene, dietary fiber, so it worthwhile to eat.

it might be "sudachi",すだち. it has very strong flavor like lime.
https://www.google.co.jp/search?q=%E3%81%99%E3%81%A0%E3%81%A1&biw=803&...
you can't peel it by hand. it can be used to add specific flavor and sour taste. generally used by squeezing, just before eating.

There is indeed green mikan, but as Ken suggested it could be any of the numerous citrus fruits in Japan. Post the photo of that sign if you can. Use a photo site and post the link.