You can flop back and forth from index to full color mode.
Tho this is very true:
Going into indexed mode can screw up your palette ramp organization
Another good way to do this without indexing or selections is use the Replace Color tool.
It works just like the Pen tool, except you can only paint on your 2nd color with your 1st.
Just pick a big brush shape and swipe back and forth a bit.
Color replace done.
It's really good for other stuff as well.
You can use it like a simple mask, or to paint continuously behind objects.

Logged
And knowing that it is, we seek what it is... ~ Aristotle, Posterior Analytics, Chapter 1