Weird monitor colours:
(I'm putting this here in case you don't check your guestbook or profile page for replies to replies, which are not highlighted by PB.)
1. It is important to have your monitor calibrated to correct for its inherent colour cast. Luckily, this doesn't need to be done often. Try to borrow a Spyder from your local camera store or photography club. You'll be amazed at the difference. It's less important at our level to calibrate the printer, but the printer driver does that, although use the correct ICC for your paper when you print.
2. Colour spaces. Your 550D can capture images in two colour spaces (think "box of colour crayons"). The first and default is sRGB, which consists of the 216 colours shared by mac and pc systems and used for the web. Better is Adobe (98) RGB - colours will be purer and more vibrant. Check in your 1st or 2nd menu page on your camera (I also use Canons). However, remember to convert your Adobe RGB images back to sRGB for posts to the web. In Photoshop, save for web and devices and check the convert to sRGB option.
This may sound needlessly finicky, but it really does make a difference.