I once coded on an HP 9845 which could change the color of the text you were typing by means of colored keys across the top of the keyboard. I used these colors for exactly this purpose.

I know it's not automated the way you want, but it became kind of a motor skill to change the colors as I went along. I didn't even think about it after a while.

You could do this easily in Word using "character style" keyboard shortcuts.

Even easier is to add an Xkeys pad so you don't have to remember where the colors are on the keyboard.