Is it ok for nurses to color their hair unnaturally (green, for example)?

There are many nurses where I work that have tattoos and different hair colors. It reflects the community where I live. If it is not prevalent in your community, perhaps it may not be as accepted…?

It all depends on what area you are working. In psych and addiction treatment it is sometimes allowed as are visible tattoos. Even in hospice but that also depends on the specific patient population.
As nurses we are tasked with making our patients feel safe and comfortable. For many older patients hair colors not found in nature are off putting.
If I weren't a nurse my hair would be pink 😉

No, this is very unprofessional, especially working in hospitals. Most places have a dress code. If you expect to be taken seriously as a nurse, keep it professional.

I work in a conservative environment,so,no