It is easier all the way around to use matching thread in the top and bottom. But that doesn't mean that you can't use different colors, you ust might have to adjust your tension a bit :wink:
Just a suggestion, try using a very narrow zigzag stitch...narrow enough that it just barely wobbles back and forth. It is a very forgiving stitch, so if you go in and out of the ditch, it is not as noticeable :D
Also, lengthen your stitch just a bit, it doesn't need to be as short as for piecing :D
Most of all? have fun :D:D:D