In the game of rook, choosing your trump color after getting your 5 card kitty is usually fairly obvious.  For the most part, you are going to choose the color you have the most cards in, unless they are very low (like to the 10 or something).   Otherwise, if you have 7 or more of one color, that is likely going to be your trump by shear numbers.   Every so often, it is worth choosing your second most color when it is extremely powerful.  For example, if you have the 11 through 1 of one color, than that is all, but then 7 of another suit, not as powerful.   You are taking a risk that you will have less trump than an opponent, but it is still possible that you can pull all trump in 3 or 4 tricks.

There is always a significant portion of luck involved when choosing trump.  There is a slightly higher probability that one of your opponents has a large amount of your trump rather than your teammate.  But what it boils down to is choosing a color that you can hope to force your opponents to play all their trumps early on without having to loose to many tricks. 

Depending on your bid, you may be willing to lose a certain number of points in your trump suit.    Count points to make sure you are going to make it.