My favourite method of creating a colour palette is to copy nature. Take a photo of something you like and extract three colours. I find a light colour for walls, a medium tone to use on furniture or curtains and a darker colour as an accent works well. If you feel you need more variation you can simply lighten or darken any of your chosen three colours by adding a touch of black or white. It really is that easy.
One thing to consider though is what time of day do you tend to use the room? Warm colours (reds, oranges and yellows) appear darker in reduced light while blues and greens tend to look lighter.
Thanks so much for sharing that simple yet oh so fundamental tip. I shall now use that tip to help transform my downstairs living space. Consideration also to what you say about the time of day (night) you use the room. Really brilliant tips and not the impenetrable waffle that you can get from other design blogs!