This is what I know so far, the easiest to catch is a green circle next is the orange circle which is harder and red which is the hardest to catch. The catch rate does depend how big the circle is and if you get it in the colored circle or not. If the colored circle is big and you get it in the colored circle then you get a "nice throw", if the colored circle is about half of the regular white circle then you get a "great throw" and if the circle is the smallest it can get and you get it right in the colored circle then you get a "excellent throw", and the and for a "nice throw" you get 10xp bonus, if it's a "great throw" you get a 50xp bonus, and if it's a "excellent throw" you get a 100xp bonus. I'm not 100% sure but I think if you get a curved ball catch then I think it also increases your chance of catching it, also you get a 10xp bonus for a curved ball catch.