In my novel, the protagonist starts out as a runaway slave, and I want to point out to the reader that she is avoiding those who are in charge of slaves who all wear a uniform involving a blue shirt and brown leather jacket. I have her travel through a busy crowd, and I want to show her being paranoid whenever she sees the colour blue. However, I do not know where it would be sensible to describe the uniform of these men. Do I include it where the characters are first introduced (which is currently in a flashback to the MC's childhood)? Or would it make more sense to include the description later when they are being spotted by someone due to the distinctiveness of the uniform?
We can't tell you how to write your story, but this is a kind of question that comes up from time to time, the sort that asks: "What is the correct way to do this thing?" Making a rule for yourself about where to describe a thing means that you're making your fiction formulaic and consistent. Good prose has life to it, it's not a list of people, places, events, and descriptions.
Descriptions are a writing technique like any other. They can set the stage and ground the reader, sometimes by conveying atmospheric information and sometimes by giving detail important to the plot. While the plot in this story demands some amount of description, please take care not to over-use it or use it in a predictable way.
From your question, it sounds like you're going to be in this character's head more than a little, so it's important to ask yourself how she would view the world. She'd live in fear of those who could return her to slavery, of course, and she'd search for these distinctive uniforms. But the reader can find out why this is the case anytime in the story. You do not need to set up cause and effect like ducks in a row in a story, you just need to keep the reader satisfied and interested.