Usually when you thread your machine you use the same color for the top spool as you do for the bobbin. This is fine when the fabrics you are sewing together are the same or when the underside of your project will never be visible. However, if you are sewing something like a place mat and the top side is one color and the underside is a different color, it will look more professional if you match the thread on each side.
This is easy to do: what you need to know is that when you’re top-stitching the thread you see on the right side of the fabric (the side facing up to you) is coming from the top spool of thread and the stitching underneath (the side facing down while
you’re sewing) is coming from the bobbin.
Match the bobbin thread as closely as you can to the fabric on the underside of your project. If there is great contrast between the top and bottom fabrics (as seen in my example) some of the darker thread will still be visible on the underside but it will be a lot more subtle.