The wrath of the lich king wiki has a great write up on Inscription, an alternative 1-300 guide and a list of what herbs can be milled into what pigments. Currently ALL outland herbs mill into the same pigment, so stock up on Felweed while they are still cheap:

http://wotlkwiki.info/index.php/Insc...al_Information
http://wotlkwiki.info/index.php/Insc...n#tab=Reagents
http://wotlkwiki.info/index.php/Guide/Inscription

You should also note that if you are planning on buying herbs "cheap" before the patch, you may already have missed the boat, my analysis from the weekend showed that herb prices are up 63% average on Silvermoon (EU) and 58% on Kilrogg (EU) in the last month.