Only in some dialects. They can just be a soft airflow with a slight constriction of the airflow. No vibration necessary.
Here groen is pronounced several times. I think this is the average pronounciation a very slight short vibration (a tap/flap).
Some people growl but as mentioned other people pronounce it even softer than the guy in this recording. (So that it sounds really close to hoen).
That site is very good btw!! Very good dialect free pronounciations but also good explanations. (I used to advise this one for pronounciations and another for explanations but by now I've found a lot of errors on that other site (dutchgrammar) and I didn't realise before that the heardutchhere site is much bigger than I thought! And also ex plains the grammar (and other things) I originally knew the page where he teaches you all the sounds)
Well if you say it's green it's clear it's unripe but doesn't have a depeer meaning in that case. A green banana is an unripe banana. (Ofcourse not for things that are naturally green even when ripe)
However you cán say it about persons. Usually as a noun and not as often as the (predicative) adjective though it's possible. You would call someone that is onervaren/ inexperienced green.
Een groentje is a rookie. I believe English has terms with green too (greenhorn?) But I will have to look them up. (Ok I did greenhorn is correct I didn't immediately see other nouns like it)
Groen comes from groeien (to grow) so it makes sense that besides the color it also means experience/maturity
If it's after a noun it's called a predicate. And in Dutch those words aren't inflected.
De appel is groen
De groene appel
Een appel is groen
Een groene appel
Het boek is rood
Het rode boek
Een boek is rood
Een rood boek
(The last one isn't inflected because of what nierls mentioned. Neuter words with an indefinite article aren't inflected either.)