I often use a different colored bobbin thread when quilting - in fact more often than not. The bigger the contrast the more you may see it on the front - if someone has a way to preven that I'd love to hear it. I figure we are just right up on top of our projects and this is simply not noticable to anyone else. Step away from your quilt and see if you can tell anything about the stitches!
Welcome from south Texas!