You may need to calibrate your Mac's display to match the display on your camcorder's monitor - or whatever else you normally watch your video on.
Go to System Preferences>Displays and click the 'Color' tab, and choose 'Calibrate...'
Here's what Dan's incredibly useful "iMovie FAQ" has to say about this..
http://www.danslagle.com/mac/iMovie/tips_tricks/6008.shtml
Alternatively, before recalibrating, still using 'Displays', just try changing the resolution of your screen display back and forth a couple of times, and see if that resets the colour and quality successfully. If it's on an inappropriate setting (..for instance if a lower-resolution monitor has been plugged into your laptop..) then both the sharpness (resolution) and colour will suffer.