That called chromatic aberration, or CA for short.
The eyepiece that came with your scope isn't made of the highest quality glass.
http://en.wikipedia.org/wiki/Chromatic_aberration
There is also Atmospheric Aberration. That's what causes bright stars, when they are close to the horizon, to twinkle madly, changing colour from red to blue to green to white to red. Very pretty really. AA happens because you are looking at that star through a lot of atmosphere and smog. Stars are always sharpest when they are straight above your head.