At the risk of over-simlifying, you have to find someone willing to hire and train you.
Now, here are some of the skills/attributes I believe are necessary to be a good colorist:
1. The ability to listen and understand what the cinematographer wants; obviously, there is no one right look and you’ve got to deliver what the director and cinematographer need to tell the story. Of course you have the opportunity to take things further than they’ve imagined, if you’re good enough.
2. A great eye for color as a design element and an understanding of the psychological aspects of color.
3. An uncompromising demand for technical excellence; you can never deliver a job that is out of specification.
4. Remarkable people skills; you will be working long hours in an environment that is sometimes highly emotionally charged. Projects take on a life of their own, they are much more than a piece of film or digital content; they are someone’s baby. Furthermore, you’ll probably hear a lot of life stories in the telecine room so you’ve got to part priest or bartender. Sometimes you even have to be a mediator when there are differences of opinion among the clients.
5. A broad general knowledge of both fine art and cinema; you will often be asked to deliver a look that is derivative of a painting or film.
That’s all I can think of for now. Some of my fondest associations in this business have been with colorists, and camera assistants too; I no longer shoot film, so I rarely have a dedicated AC, and we do our color grading in-house, so no more hours in the telecine for me.
Best of luck in your quest.
Leo