The great thing with Kmail is that you can manage a work flow with the 'Important', 'read/unread' and 'todo' tags. However, the priority currently in which tags should color the message text are in my opinion wrong. Currently, when you set a message as important, the text color is red. When you set a message as unread, the text color is blue. And when you set a message as todo, the text color is green (on my configuration at least). Unfortunately, when you set the todo tag on an unread message, well, you can only see the text as blue, not green. If you have an important message that imply that an action is necessary, setting the todo tag will not help since the color will stay red, not green. If you receive a message, starts to read it but mid-way find out that it's an important message that you will have to postpone reading for later, set in unread again and set the important tag, no luck, the message color will stay blue. So currently the tag precedence are unread > important > todo. I think it would be more useful to have the following precedence : todo > important > unread.
I think a better solution to this problem is to use a different font style for unread and todo messages, as I proposed in bug 382987 because of poor color contrast. If unread messages are listed in bold, and todo in italics, for instance, there would be no issue concerning color precedence. An unread, important, todo message would be bold, italic, and red (according to Alexandre's color scheme). In my opinion, one should *always* be able to tell at a glance how a message is marked, and the only way to do this is to use font style instead over color.
It's true that ideally, you should be able to quickly know how messages are marked. However, just using bold/italic is not enough in my opinion to distinguish marks between them, specially if you quickly browse your emails using the mouse wheel to see if there are any outstanding TODOs. Color in that case is more visible.
I haven't had a problem distinguishing bold/italics from normal style, even when quickly scanning the mail list. Thunderbird uses bold for unread messages. Italics might be harder to distinguish, though I've no problem with it, but I think the compromise is worth it--sometimes you want to mark something both as important and an action item, and it would be odd if you could not see that at a glance but would have to infer it somehow else.