Colour, color, couleur, colore - no matter how you spell it, if your a designer you need to know how to work with it.
Thankfully there are some great sites out there to help you out with that. Here are a few of my faves:
Colours on the Web - This is one of the best and most helpful colour sites out there for using colours in web design. From info on the basics of colour theory to providing the technical tools to help you work with colour including hue, saturation, tint and shade variations for hex colour values. The colour wizard also displays colour palates for monochromatic, complimentary, tertiary and more.
Color-Hex.com - Plug a hex value into the box at the top or set one from the pop-up colour picker, click "get color info" and see it viewed in the background of the page you are on, in a button sample, as a border and much more. It even shows you the RGB breakdown of the colour which can really come in handy.
Colour Combos - A handy little site that includes a colour tester where you can enter and view multiple hex values to see how they work together.
ColorPicker 3.1 - Helps you choose your colour palette for backgrounds, text, hover and links as well as giving you the CSS styles and hex codes for the colours you choose.
HTML Color codes - A simple listing of colours, their hex codes and sample colour swatch.
Design Seeds - Pure colour inspiration. A simple but spectacular blog of beautiful photos and the colour pallet they contain. Pure genius.