As a question, did you print using a CMYK colorspace or an RGB colorspace? CMYK (Cyan Magenta Yellow blacK) is better for printed applications, whereas RGB (Red Green Blue) is better for video/monitor/electronic display purposes.

Basically what happens is in CMYK (subtractive), if you mix all the colors together, you'll get black. If you take yellow ink and blue ink and red ink and mix them all together, you'll get a very dark color. If you add more ink, the mix gets darker. In RGB (additive), if you mix all the colors together, you get white. It's harder to visualise, but when you add the outputs of the red gun, the green gun, and the blue gun in, say, your old CRT monitor, you'll get a very light color. Add more light, get a lighter color.

In Photoshop (and should be in GIMP, as well), there is an option that allows you to convert from one colorspace to another. Being in RGB while you were printing might explain why your colors looked so much darker when you printed.

I hope this helps!