The reminder circles are the color of the category as set in the Reminders app.
For instance, I've got purple for work and green for personal tasks.
My colors messed up for some reason, I had a reminder set as GREEN, but the color 'assigned' to it was purple (the default).
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When getting a message or notification (such as a 'text' in Kik Messenger) the lockscreen flashes white, black, and then displays the notification in a normal manner. The flash is VERY fast and just barely caught my eye, but noticeable when looking for it