What is the difference between a yellow star rating and blue star rating?

A yellow rating is your rating.  The blue rating is the average of all the ratings thus far.  If you are seeing a blue rating, you personally have not rated that particular ISBN.

Just to help out with people rating the books you've read (which is highly encouraged!):

Yellow stars are the rating you've given a book.  You can change this at any time by scrolling your mouse over the stars and clicking in the new place to change the rating.  To remove all your ratings, just click the red slashed circle after the ratings to erase all your ratings for a book.

Blue stars means you've not yet rated a book and the number displayed is the average rating given by other members.  The number at the end of the stars indicates how many other members have given a rating to this book.

And another note:  you must rate a book before it can be added to your BIR (Books I've Read) list.  Rating a book automatically adds it to your BIR list.

I think you also need to rate the book before you can post a review.  At least that is what I was prompted to do when I wrote a review.

Yes, you do have to rate it to review it, but there's a way around that. If you rate and review it, then erase your star rating, your review will still stay up. This is useful when you have something to say about the book, but for any reason don't want to rate it. Maybe you don't want to effect the average, or you don't want it in your BIR, or you simply don't feel you can rate it fairly. I've done this a few times, when I wanted to comment on something about the book, but I didn't feel that I could rate it as fairly as I would like. For instance, if I couldn't finish a book, or it was a book that was difficult to rate at all, then I'll review it but erase my star rating.

Why would you review a book you haven't finished? I DNF a lot of books but I don't review them--I don't feel qualified to do so since I didn't read the whole thing. Mostly it's just that the book is not something suited to me. I make a notation for myself on my book journal so I don't pick up the book again, but that's about it.

I get quite a few ARCs to review, and even if I thoroughly hate them I make myself read those to the end IF I am going to submit a review. If I absolutely can't finish it, then I just let the person who sent it know that I won't be reviewing.

Cheryl