Garter Miter Seams, Multicolored Edition

Hi y’all—Where better to get advice about miter technique than MDK? Question. I’m making a garter miter afghan where I stripe the miters so that different colors are meeting up where I need to seam—like Kay’s stripes here: Now, it’s been a while since I did mattress stitch in garter, but my memory is that … and hopefully my memory is wrong … the seaming yarn doesn’t disappear as much as it does when you seam stockinette. So I don’t know which color yarn I should use on the seams. I certainly don’t want to change yarn colors every time I hit a stripe! Am I overthinking this? Advice requested!

Here’s mine seamed in another color (3 needle BO if I recall correctly.). Just to see it. The darker color is very subtle in person.