Stitch n Bitch Go-Go garter stitch scarf-Add new yarn help :/

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Hello all, maybe someone can help? I'm new to knitting, I'm teaching myself off the SnB book. I'm making a striped scarf. I have added the new yarn color, but why am I getting the line of where the other colored yarn starts on the opposite side? Here is a picture so it can actually make sense hah.

I folded the scarf so you can see both sides.