This is how I learned color: I wear the suit and tie to the clothing store, and I grab an armload of scarves, and I hold them up to the suit and tie, and I go, "Does this look funny?" If I'm not sure, I ask a female passer-by. (I deduce, from their reactions, that women love it when random men in suits ask them if a scarf goes with a tie.)
Here is an example from the suit/hat pairing I wrote about:
The tie is a JAB number that brings out both the rust windowpane and the white windowpane in the suit's fabric while adding a consonant blue. The scarf, meanwhile, picks up a lot of the same earth tones as well as the black present in the suit's brown. It also goes well with my camel-colored overcoat (apologies for the inaccurate color balance):
Some general ideas:
1. Families of colors. The ensemble above is all about earth tones--brown and red. So various shades of brown and red are safe. The key is not to get the shades too close together. If it looks like you were trying to get identical shades of brown but you failed, the effect is really distracting, like simultaneously listening to Middle C and one hertz above Middle C. The dissonance is inversely proportional to the distance between the colors.
2. Pop colors, and inverted pop colors. The idea is to take a minor color from your tie and make it the major color of your scarf, or take the major color of your tie and make it the minor color of your scarf. Solid red tie with just a touch of navy? Go with a navy scarf. Or go with a scarf with a hint of red and colors similar to your suit. Gray tie with purple stripes? Go with a purple scarf. Or go with a multicolor striped scarf whose dominant color is purple.
3. Neutral overcoats. Grays, blacks, and tans will maximize your flexibility when it comes to picking scarves.
Finally, remember that your scarf may well obscure your tie when you're out and about, so if you see a scarf you love but which doesn't really go with the tie, be adventurous. Sometimes color coordination happens by accident.