I use category colors to manage my ridiculously large flow of incoming mail.  When a message arrives, I have rules that set categories based on how the message was addressed.  For instance, a rule for messages sent only to me (nobody else on the To or CC lines) sets the category and color of the message to red.  A rule that sees me on the CC line with others on the CC line sets the category (and color) to green and so on.

What I noticed is that every email that has an attachment will briefly change color appropriately when it arrives and then the category is cleared shortly after.  Usually within a second.  My theory is that my rules set the category correctly when a message arrives but that the category is cleared after the attachment is completely downloaded.

Is this accurate?  If so, is there any work around I can use to keep my categories set as I want them to be?