I read it roughly 1999 but it was a tatty old paperback, I can't remember anything about the book cover. The story is a small team of 3 or 4 who work on commission to ensure some order in the galaxy by whatever means necessary. I think the team was a girl and an ex military guy plus an older guy and also a very flamboyant guy. He was like a cross mixture of Boy George plus early David Bowie, with the fashion sense of Cat from Red Dwarf.
I got the feeling that the book I read was 'mid-series', the main plot section I remember is they had to assassinate an Empress of a cluster of habitable worlds but she had very tight security. They faked up they were Ambassadors from another star system and got invited to a Ball at her palace.
Flamboyant guy was from a world where male beauty was highly prized and he glammed up at every opportunity,he went OTT for this occasion, the macho guards were amused as he arrived in a gold dress and covered in gold body makeup, he minced up to the Empress and kissed the back of her hand, leaving a golden lips imprint there, he said it was a sign of fealty so she left it on her hand to show off.
An hour later he left, one hour after that the slow poison from the lipstick reached her blood and she fell dead - by that time he'd scoured his mouth and taken the antidote. That scene is really all I have, possibly it was a fix up book with each chapter being another story but I'm not 100% about that.