tedster - 4:21 am on May 15, 2012 (gmt 0)
is href="blue.html" something else?
That URL indicates a resource that's in in the current directory - the directory where the current URL exists. So if the current URL is in the root directory, then there's no difference. But if it is in a subdirectory, then it is VERY different.
I'd suggest you experiment on a local version of a website, using all kinds of configurations. This will help set the variations more firmly in your comprehension.