ColorSync profiles can be your best friend - or your worst enemy (mostly the latter). There are a lot of good resources for learning and understanding how to set up, calibrate and use these profiles online. In a nutshell, if you're not trying to match your printer to your monitor and only want to control monitor images, make sure all your RGB profiles are set to "Monitor RGB". It's the only way to asure WYSIWYG images!
In another note, the Preview app never displays acurate color and can't be trusted. It should be punished.