The colors can change en route if the target changes course. In a tutorial, make sure you are pretty much at a right angle to a target that's not changing course. You may want to lower your scope and decrease speed to avoid detection. Like the man said, open torp doors almost ommediately and approach woth them open. Aim slightly forward pf amidships, kinda like leading a bird.
If an old man like me can do it, so can you!
