Mine seem to form organically over time, but I spend a very long time on my stories before they become anything tangible. Have patience with yourself and your characters themselves.
Nearly every character I have ever written has taken on a piece of my own personality or is based on an experience I had. Sometimes they have pieces of my friends in them, too, or other people I've known. When I'm trying to flesh out a character I usually start with what their purpose in the story is (are they a main character? are they a bg character? are they a romantic interest? are they an antagonist? do they represent something in the theme or plot?) and then I move on to their visual design (shape language, colors/how I'm going to use color in the story, how do they dress, etc.)
Some characters come a lot easier than others, but its not always the important ones that are easiest; sometimes if I have an important character that feels really flat i will try to make them a playlist or imagine them outside of the plot in their own lives.
When minor characters are flat I tend to allow them to stay that way until they become convenient to the plot or I can incorporate a gag or pun into their being. Sometimes things just become important later, when you never intended them to be in the beginning. This is the most fun part of writing in my opinion.
I also try to keep things like names and colors very Plot Economical, in that they will usually have some important meaning in the context of the story. But not always, since sometimes a name is just a name. Sometimes they also serve as a mental shorthand to remind me who this character is and something important about them, even if it doesn't always become critical to the story.
That got really long, but I hope that it helped you!!
