This is merely a half-assed attempt to demonstrate how to make an image using GD and an input form. Just to muddy the waters, I threw in some hex to decimal and vice versa conversions copied from the PHP.net
In this example, the image is written to the "cache" directory, which needs to be writable by the server, and then eventually flushed by a cron script.
If you feel like playing with it, just view
the source and copy it into an editor.
(if you enter a hex value, the rgb values will be ignored and overwritten)