Is It Legal to Paint a Picture of a Celebrity?

I want to do some paintings of Marlene Dietrich, I'm wondering if that's okay? I will not copy photos or video stills exactly and they won't be completely photorealistic but I may use the same hairstyle and outfit. So how would I have to do it to make it legal? Thanks.

Suggestion:

as long as you don't try to pass it off as someone else's work, perfectly legal!