The spritefont object can't have some of its characters recolored temporarily. There is really no easy way to that without resorting to some clever workaround.
You can map additionnal characters that are almost exact "copies" of existing ones and are mapped to differently colored characters. To create your additionnal characters set, you can use something like this : lingojam.com/WeirdTextGenerator
In your SpriteFont sprite, you then add additionnal characters to match your new characters set and apply whatever color you need.
For example :
abc... are mapped to white characters
๐๐๐ ... are mapped to red characters
๐ถ๐ท๐ธ... mare mapped to green characters
etc.