Establish Trust and Confidence
When God (or whatever your higher being of choice is) created horses he made them “fear runners”, meaning when they are scared their flight instinct comes into play. I suggest you take extra time in your first encounters (petting, brushing, even feeding peppermints) with a colt to establish a trust bond so they are aware you are not a threat to them and will not put them in danger. Once this is established a vast majority of colts will relax and be willing to learn and progress in your training program. Throughout your training with each colt be sure you are moving about them with caution but also an air of confidence as horses can sense nerves and fear.
Let them tell you
When you advance to the lunging/long lining part of your program watch them closely. Watch their actions and reactions to different things and how they carry themselves. Most colts will tell you through their actions and body language a ballpark idea of their ability and capabilities. Also be sure to channel their energy not subdue it.
Take Your Time
Training is like a pyramid, if you don’t lay a solid foundation it will not last the test of time and come back to haunt you. Be sure they are comfortable with each step of the process before moving on to the next step. It is much harder to go back and have to re-teach basics later than taking the extra time and doing it right initially.