Is it possible to ask the SpriteFont if it's between letter number X and letter number Y, and change the letters inbetween those numbers to a color or whatever, because whatever I seem to try either colors all the text or even crashes the game (older version of SpriteFont though, still, and could be me misusing some expressions).
Failing that, can you make it start recoloring text by first seeing a symbol like [ which isn't in the graphical alphabet sprite (which I assume would make a zero width sprite?) and then maybe ] to make it change back?
EDIT: What I tried last that just colors all text is "For Each letter" ---> "SpriteFont.LetterPositionInPhrase Lower or Equal to 50" ---> "Set Font sprite opacity to 50", then just switch it back when it's over 50...