Dado has a lot of experience in the field, but no course will turn you into a “professional colourist”
Practice, practice, and more practice, using real word footage, in real world situations, with real clients making real (sometimes outrageous) demands is the only way to become both proficient and professional.
Darron has also run a paid course a few times. It is a bit pricy, but he does take you through actual projects, with actual footage. The last one was 5 days, for about 4 hours a day (Live), with the ability to watch a recording later. He does not sell these after the fact, they are only available to those that sign up for the live sessions. I’ve been colouring since 2000, and learned some great techniques from his course. We all have different ways of doing things, and it is nice to see someone else’s approach.
Far too many courses you will see on YT are based around single shots. “lets fix this shot…now here is a completely unrelated shot…lets put a look on it” These are typically useless for a day to day working colourist scenario. Look for courses that take you through a project from start to finish.
Preferably one that will let you use the footage for your reel afterwards.
Mixing light.com has some practice courses for sale as well, that you can show prospective clients what you did.
There is also some editing courses that will let you have access to the raw footage for the same purpose. I have some of these for practicum students I house, with the understanding that they are allowed to put the finished projects on their reels.
Look for editstock.com