Welcome to KH!
Merigold explained it well ... the "stripe" that you are talking about just happens naturally when you change colors. Usually it's on the wrong side of the work (eg. on the inside of a sweater) so you don't see it. With a scarf, there isn't really a wrong side since you see both sides. I don't think it's really a big deal though.
In the future you may want to try one of the more advanced techniques that Merigold mentioned, but it may be a little overwhelming right when you're just getting started. I like the idea of doing your color changes on different sides so that the "stripes will alternate and be on both sides of the work ... it will look like it is supposed to be part of the pattern!
Anyways, hopefully you find the forums here helpful and good luck with your project!