As far as I can see there is no query (search) syntax to select current note to be able to define color in Graph color groups.
I strongly suggest to introduce one, so users can set a special color to the currently active note.
I think the intended use is to assign colors to query results and not explicitly to notes. That is, you don’t tell Obsidian to show this note in green; instead, if this note matches the green criteria, then it will be shown in green.
Having said that, you can create a system of “color tags” (#color/green) and “color queries” (tag:#color/green) and then apply those tags to individual notes.
In this post I want to share with you a color tool that is CMYK to Pantone. This is very easy to use and it’s free. In official version it’s paid but we are create a free version on this.