The top thread and the bottom thread should meet in the middle of the fabric. I would do another sample with two pieces of fabric. Maybe you could see it better that way.

In my opinion, the darker thread stitch doesn't look as good as the lighter colors. I was told that this was because of the dye process necessary to get the darker thread. Darker thread has a fuzzier look.

And with two colors of thread - so the fabric and two threads are three different colors.

When you get it the way you like it, take a picture with your phone. I do that with projects with certain settings that took me awhile of fiddling with. So all in one photo are the picture of my sewing machine with stitch width and length on screen, along with fabric/project working on. Saves a lot of time .

Good advice! not only mark your adjustments, but make notes to yourself as others have said. I have found that non-English-speaking do not know how to write clear instructions, so when you write yours out, you will be in better shape to remember what you did.

They type and size of needle can also effect your stitches...I have found that certain needles do not always play nice with different fabrics and sometimes my stitches look uneven. Just a thought.

I found this post to be very helpful . I'm not having problems right this minute but I have in the past. All great info. Thanks everyone!

Yes -- many, many thanks for all your help!

Make a notebook of all your samples and keep detailed notes. Different fabric, different threads and different needles really can change things. Finding the right combination is no easy task. Always test with the combination of layers of fabric and batting that you are using. Again, fiber content changes things. Too bad we can't just sit and sew without all the hassle. I guess that is the reason for all the dials and screws!

The stitch definition looks good. If you aren't already using a single stitch needle plate, that might help a bit along with using more starch/sizing before you start sewing. The stiffer the fabric, the more the thread will sit on top, rather than sinking into it. But the black thread on a shade of green might also be throwing off how clear the separation appears. You might try using the same thread with a pure hue, tone or tint for comparison so the black really stands out.