This activity has more benefits than just the aesthetics of the outcome. You get to touch and look at books you may not have paid any attention to since you put them in the bookshelf. Be warned the process takes quite a bit of time but is really enjoyable. So make sure you are in a happy mood and have an afternoon to spare (depending on the number of books you own). I find it helps if your books are already in a bookshelf so you can see all the spines easily. Simply start by taking out books of the same colour and stacking them in colour piles on the floor.
Your bookshelf design and the number of coloured books you have will force you to be creative about how you display the books. ‘Cube’ shelves as pictured are easy for separating individual colours. But if you have long shelves, or even one very long shelf, you could blend the colours into each other. I’ve even seen a rainbow effect of books. It looked amazing.
Personally I feel better about life when my books are colour coordinated.