Switching once?

• Once per paragraph?

Sort of. Use a Character Style. Apply either Black or Automatic as the color in that style. You can attach that to a keyboard shortcut. To turn it off, Ctrl+Spacebar reverts to underlying paragraph font settings.

• Once per Word session?

Nope.

If you were using a word or phrase always at the beginning of the line, this could be done combining a multi-level list and a style. See: this thread: Nested Styles in Word. The word or phrase becomes part of that style in that document.