I've seen hairlines of color before in a module and it's usually got something to do with the HTML structure of the email or where the colors are placed in the HTML. In my experience it's when the red color is set as the background and you've got other colors overlapping it but it still shows thru at the top/bottom on some displays for some reason. Maybe try playing with which element (tables or td) you've got your colors placed into and try removing a little bit at a time to see if anything will resolve the color issue.
If you're able to share the html for that specific module/section we might be able to find some clues in the HTML structure, but it might also be something in the <style> in the <head>.
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