Color coding your email is a receiver side option. You can do this via the Organize or Automatic Formatting feature in Outlook 2007 and previous or via the Conditional Formatting feature in Outlook 2010.
Eventually it will be up to the receiver how he/she chooses to organize his/her mail and make sure the received emails get the proper attention they deserve. You can help/influence this sorting process via other methods though;
* Flag the email for follow up by the recipient.
* Set a priority; Low, Normal, High
* Set a sensitivity; Normal, Personal, Private, Confidential
To do so, Select the Options tab and click on the expand icon of the More Options group.