George R. R. Martin's Song of Ice and Fire series has received much attention and accolades, and on the whole they are deserved. It's a marvelously ambitious story, though the volumes are getting sparse and I do wonder if Martin has overreached. This story, The Princess and the Queen, is from the history of Westeros before the time of The Song of Ice and Fire.
The title is a little misleading, since the focus of the story is a succession war between a king's designated heir, a princess, and his son. But it's explained in the introduction to the story as the "blacks and greens" part explains the rivalry of the princess and the prince's mother.
This story is not any sort of introduction to the series, so if you're looking to dabble this is not the way. It's also not an extension of the main plot, for the further entertainment of the casual reader. This is a history lesson, written pretty much the way European history is usually written. Lots of names of royalty and battles. Pretty much everyone loses, which is the object lesson.
This is the sort of backstory that Tolkien, Robert Jordan and any other large-scale world building fantasy author has to write in order to make sense of the main line. Martin found a place to put it out for public consumption. I recommend it only if you are an obsessive fan. As a somewhat more casual reader, I found it more of a slog than entertainment. There are no significant characters overlapping the Song of Ice and Fire, but lots of shared names or close analogs. So someone by the name of Viserys or Rhanys will be a player, but not the ones we remember, so it ends up pretty confusing. It's pretty dry stuff--very much the historical read. I'll give it two stars, but only because I know that for the true otaku it will be worth reading. Like the Tolkien backstory volumes. Knock yourself out.