customize mailbox icons in Mail
The BAD news first... Until Apple decides to include such a feature in a new version of Mail, there's NO WAY that I know of to make each mailbox icon different than the other.
The GOOD news... YES, there is a way to REMOVE thos UGLY blue folders. You'll end up having mailboxes with no icons. I personally like the look very much (at least, much more then generic blue folders). To do this, follow these SIMPLE instructions:
1) Go to your Applications folder and create a COPY of your Mail.app (just in case you need it later) and save it someplace safe. To do this DRAG it to your desktop while holding down the option key.
2) Now control-click (or right click) on your Mail.app (the one in your Applications folder) and select "Show Package Contents"
3) Open the folder called "Contents"
4) Open teh folder called "Resources"
5) find the file called "alphaPixel.tiff" and make TWO COPIES of it to your desktop as well (just like step #1).
6) find the file called "NormalMailboxLarge.tiff" and RENAME it "NormalMailboxLarge(original).tiff"
7) find thefile called "NormalMailbox.tiff" and rename it "NormalMailbox(original).tiff"
8) RENAME one copy of your "alphaPixel.tiff" (which is on your desktop) to "NormalMailboxLarge.tiff" and place it in the "Resources" folder.
9) RENAME the 2nd copy of your "alphaPixel.tiff" (which is on your desktop) to "NormalMailbox.tiff" and place it in the "Resources" folder.
10) Shut down Mail (in case it's open), CLOSE all windows, then START Mail and Voila! Your mailbox icons are now just text (much more good-looking in my opinion)
CONCLUSION:
Yes, one day Apple will realize that many of us like to customize our Mail's mailboxes as per their purpose/theme, etc. But until then, if you alter the look of Mail's blue folder mailboxes (as described above), you change ALL the mailbox icons. Also, REMEMBER, if you want to change the look of the "smart" mailbox folders in Mail, you will have to do the same thing as above (you will have to find the "smart" .tiff files and replace them with whatever .tiff icon you prefer). Good luck Mac brother (or sister ;-)