The arrow changes from Green and up (best performance), Blue and across (average performance) to Orange and down (not so great performance). When you have more than one Bob you can choose the one with the best performance, if you only have one Bob you are stuck with how he performs.
There are lots of threads on this forum discussing these arrows. Have a look around you might find some useful info.
Try searching for "arrows" using the search function just in this section of the forum.