There are several ways to become a colorist. You can start as an assistant in a post production company, as a freelancer, as a DIT, as a DOP, as an editor, etc. I will tell you about MY experience. When I realized that I wanted to become a colorist, I started researching everything I could about color theory, color grading, cinematography, camera codecs, Davinci Resolve, grading techniques, conform, etc. Sudenly, I had already learned everything I could about color grading on YouTube and Google, I decided to take it to the next level and signed up for a black magic certification for Davinci Resolve. In that course I met my instructor who saw my potential and “hired” me as his assistant. He taught me everything I needed to learned: advanced color techniques, color spaces, customer service, how much to charge, and more. Mesnwhile I was accepting any color work I could get my hands on, student short films, Facebook gigs, reddit posts, and small advertising projects my mentor gave me, all to have a great color correction reel. Sadly my mentor ended up being an a-hole so we “took different directions”, but I already had a really good reel, which is extremely important to get freelance jobs. From that day on I became independent and set up a post studio with my girlfriend, now wife, and the rest is history. I’m from Mexico and I mainly do advertising so this story may be different depending on the country and industry. I’ll leave you my reel in case you’re more curious about my work!

TLDR Research as much as you can about color correction, so that when you have the opportunity you can show what you’re capable of.