I googled and found this;
[size=2]http://knitting.about.com/library/blshamrock.htm[/size]
I like a two color cast on and to get this I use a half-hitch by looping the yarn over my thumb in a manner where the yarn from the skein is over the yarn going to the needle. Put the needle under the yarn and into the loop around the thumb. To make sure the cast on is closed and not an open work with two separated pieces the two colors must be twisted at each st. This is done by always taking the new color from under the old.
To keep the edges tidy and attractive I purl the last st using both colors together and slip the first st of the next row.
Plus I think I’ve seen a two color cast on in some of Anna Zilborg’s books. She does great color work.
Good luck, mwyn