Visual Studio uses code highlighting. While you can usually customize the color scheme, you cannot arbitrarily color portions of your code.
All keywords have one color, all strings have another color, all numbers a third, etc. So if your A's and B's are the same language component, they will always have the same color.
Edit: I've just noticed that I've answered a few of your previous questions, and you never got back to them. No comment, no picking a best answer, no feedback whatsoever. Keep up the good work.