Re: How do I lighten up walls painted too dark?
You never paint an entire room in a bold color. Use color sparingly and only paint one wall as an accent to the rest of the room.
Sounds like you've got a very dark cave now to contend with. Your friends idea of a lighter and complimentary contrasting color is a good one. You can sponge the color on or use a relatively dry roller with the color in it, this will allow the base color to bleed through, yet still lighten things up.
Your only other choice will be to repaint the room in a lighter color, which should be several shades lighter than what you want the final color to be, because the dark will bleed through.