You can easily adapt the Mutt email client to support coloring of messages. I find this useful for flagging messages from people I care about and subjects I care about. To define the color behavior of Mutt, you need to add lines to your .muttrc file. The lines you add should be of the form:

color object foreground background [ regexp ]

where object is one of body, header, or index to highlight text with color in the body, header, or message index respectively. foreground and background are the color to use. And [ regexp ] is a regular expression which finds the text to which the colors are applied. Here are some simple examples:

color index red default Error
color body blue default Halo

The first will turn the text in the index to red if it is 'Error' and the second will turn the text 'Halo' to blue in the body of any messages, making both of these important pieces of text easier to find.