iPhone colour green or ultramarine?

That's completely a personal preference. There will be absolutely no functional difference. What one person here will like will be different from what another person will like, and neither may be what you would actually like. Pick what you think you'll like the best. If you're ordering from Apple, you'll have 14 days to return the phone if you decide you don't like the color.

Regards.

That is only a question that you can answer. We don’t know you or what you like. Flip a coin. Heads it is green, tails is ultramarine.