At my last workplace they started allowing all kinds of unnatural hair colors (e.g., blue, bright purple, green, etc.) along with all kinds of body piercings. Where I work now it is officially a violation of the dress code and workplace expectations but I do see it here and there. Natural hair colors, even if it's very different from what your own native shade, such as bleached blonde over dark roots, is allowed anywhere, as far as I know. Having said that, even if unnatural colors are not objectionable, consider the image you want to project as a professional, particularly if your patient population includes the elderly. My father is 93 and I know what he thinks about it. It doesn't create a positive impression with him, and is a distraction.
Depends on the hospital. For example UCSF does have more leniency when it comes to non natural colored hair where as some places like Stanford do not allow this. Check your hospital policy
