Hi, not exactly what you were looking for but you could...
Put "A" in the rows shelf, "B" in the columns shelf, then drag "Text" to the text option in the marks card, then drag the "color" to the color option in the marks card. This changes the color of the font, and you can set the colors yourself. See the attached workbook.
Hope this helps.
edit: pic added
Book3.twbx 16.5 KB