Andy, click on your User CP, then click on Edit Signature. In the window that opens, as you already have discovered, you put your text. To make it colorful, select the text that you wish to color. Then look just above it, on the toolbar. You'll see a big Capital A with a color bar underneath it. Click the little down arrow to the right of that "A" and you'll see some color selections. Click the one(s) you want, and that will change your text to that color.
Here's a quick guide: Set your link URL by: [url=http://yoururl.com]Your Anchor[/url] Your Anchor Set your anchor color by wrapping the following tags around the anchor text: [color=red]Your Anchor[/color] Your Anchor Bold or italic your anchor by wrapping the following tags around it: [b]Your Anchor[/b] or [i]Your Anchor[/i] or both [b][i]Your Anchor[/i][/b] Your Anchor or Your Anchor or Your Anchor Align your link to the right by wrapping the following tags around it: [right][url=http://yoururl.com]Your Anchor[/url][/right] Your AnchorOr center it by using the [center] and [/center] tags instead: Your Anchor So if you wanted bolded blue anchor text, aligned centrally you would need to used the following code: [center][url=http://yoururl.com][color=blue][b]Your Anchor[/b][/color][/url][/center] Your Anchor Notice that, unlike HTML, there's is no need to include quotation marks (") in the code - it just wastes characters from your 350 character limit.