Understanding colors
Some theory here:
http://www.worqx.com/color/index.htm
See the Color Scheme Generator, Color Wizard, Color Combinations. They all have some theory or rationale.
Color and humans is a very complex topic. Scientists do not completely understand how we humans perceive color. (See also http://en.wikipedia.org/wiki/Psychophysics)
There are a lot good books out there but some free resources I use:
http://en.wikipedia.org/wiki/Color
http://en.wikipedia.org/wiki/Color_vision
http://handprint.com/HP/WCL/wcolor.html
http://www.efg2.com/Lab/Library/Color/
http://www.cis.rit.edu/fairchild/
In addition to the links I'd like to post my way of selecting pleasant colors:
NEVER ever use pure colors. Even if you want a pure color, don't!. If you want a strong bright green for example don't use 00ff00. Use something like 10e013 instead.
If you have one color that you like, and you want another one that fits to the first open a graphic program. Go to the color picker, type in your color and then switch to HSV mode. Then adjust either one of Hue, Saturation or Lightness. Don't modify two or all parameters, just one. That makes sure the color you choose is perceptually related to the color you've started with.
If you have no idea what color to start with get a classic masterpiece of painting from the net. Blur it a bit and then pick some nice colors from it. If you use some common sense it's hard not to end with pleasant colors this way.
Just to give you an example: I've just picked these colors:
From this painting:
http://www.cs.nthu.edu.tw/~sheu/Images/Monet.jpg
I know - it's not exaclty what you've asked for, but I learned these tricks the hard way.