It's actually a longer story than that!
The reason I even got concerned about the reddish hue was that it was only on the Inbox, Drafts, Sent, and Trash which are on the Comcast server. And, coupled with the fact that I've been trying to debug a mail issue for the last few months, ever since I changed the mail account type from POP to IMAP (because I added an iPhone), I'm looking for any possible straw to grasp! Otherwise I would have probably written off the reddish as something I didn't know about or care about!
The mail problem(s) are that not all emails that I send get into the Sent Folder on the Comcast server. It's intermittent. And, intermittently unneeded copies of drafts of some emails get written to and stay in the Draft Folder on the Comcast Server. They don't go away til I delete them. Sometimes when I send an email it causes an Outbox Folder to be created. Sometimes it doesn't. Sometimes such an email is bigg, sometimes it's very small.
Macbook pro, Mail 8.2, Yosemite 10.10.4, IMAP account.
So, that's the long answer as to why the reddish hue peaked my interest, seemed like I should learn about it! Then, the other day I noticed that blueish hue... So, it started sounding like Diane had a good tip, i.e. reduce transparency and see what happens. I did this and will see if the red returns!
BTW - here's my desktop. It doesn't show red or blue in the top left corner area.....

Thanks for all the ideas and questioning. Sorry if all this is a red herring!