Who's this? (May be a celeb)

At a 1990 party - the lady on the left is Joan Collins, but who's the one wearing the bow tie with her tux? She looks vaguely familiar but I can't place her.

Nice pic - thanks for sharing.

Maybe Terri Garber - she wore bowties in 1 or 2 episodes of Dynasty.
The eye color is often difficult to differentiate in such shots.

Nice guess, won't say it isn't her. However, she's american and this is an event in England (where Joan Collins is from) - shame Shutterstock just names the other two.
Here's a couple pics of Terri to compare:

Another guess: Princess Alexandra Schoenburg.

She was the wife of Taki Theodoracopulos who hosted this event.
Here is another photo of her of this decade:
https://www.shutterstock.com/de/editorial/image-editorial/party-mark-fall-communism-savoy-hotel-princess-7527543t

That seems pretty damned likely! Pictures of her in later years show very similar eyebrows and smile.