as far as I am aware, most of the asylumnites use a RGB Value of 40,4,4
Start off with that as a base background colour for your canvas and build your design up in layers above that background. There are several methods of getting the transparency when saving as a gif when your finished. Important thing is to have the bg colour close so that any edge colours that are left dont stick out like a sore thumb when the gif is overlaid on the forum bg.
Read the Doc's tuts on transparency at Handson (link at the bottom of the page) This should give you a great foundation.
edit: borrowed DB's stupid fongers 
[This message has been edited by vogonpoet (edited 10-06-2001).]