By Timothy Zahn.
The Greens and the Grays are both groups of alien refugees from a war on their home planet. They've been quietly living on Earth since about 1928, until they each discover that the other group still exists. Trouble starts because, of course, the war was between them.
This book could easily be a sort of cheesy alien movie. There don't seem to any terribly deep ideas in it. Instead, there's a couple who are given Melantha Green at gunpoint and told to take care of her. And the police officer who thinks all he has to stop is an impending gang war. And the classic move of "Let's go sneak into the enemy's HQ while supposed to be out meeting with us somewhere else", only Roger and Caroline (fortunately--anyone can see what a stupid idea it is) don't manage to get that far. The outlandish explanations of the physics involved in the Gray technology and eventual plot twists (what planet did the Greens and the Grays come from, anyway?) make this story seem more like fantasy than sci fi.
Not offensive, but also not substantial. It seems kind of fluffy.