The little jog at the left is normal. If when you turned the work and started the new row, you slipped the first stitch rather than worked it, this would be less visible.
Whenever you join two colors you will have a row of stitches that is half color A and half color B. Whether those stitches appear on the ws or rs of the work has to do with whether the new color was added on the rs or ws, and what type of stitch you are working: stockinette, garter etc.
Yes, you can change the appearance of this mixed color stitch by changing whether you knit or purl a row, you will, however, also change the appearance of your overall work. For instance, if you are working in stockinette, and rather than knitting down and purling back, you decide to purl down (with new color) and purl back, you will throw in a row of garter stich. This garter stitch will be more obvious in your work than the color change, so you have to decide if this is how you (and the designer) wanted your work to look or not. If you want it to be flat, then con't the stockinette, if you don't mind the ridges, work some garter stripes. On a scarf, nearly anything goes. On a sweater, you really should stick to the pattern, and the color chnages will be hidden on the inside anyway.