This is likely NO surprise to anyone thats done much quilting. If you want the fabric underneath to stay just like it is..... a same color/value solid or variegated thread works great! Like it like it is!! This is a great choice for newer quilters when still mastering stitch skills. ( Again... thread spool shown at bottom of photo)
Variegated works somewhat similar but adds a tiny bit if interest.
Finally,lets just look at comparing three values of a complimentary thread color...
On the same mid value background red fabric... a dark, same value, and light compliment thread are stitched in similar quilt density.
Can you see a difference? It's actually harder to see up close than far away....
The DARKER value thread darkens the value of red fabric
the SAME VALUE thread does little but is a bit drab (neutralized) by comparison..
LIGHTER VALUE thread draws attention to the area and lightens the overall value of the area.
Good stuff to know... Thats it for 2013. New projects to arrive NEXT YEAR! Stay safe tonight. Happy New Year!!!