A medium gray is great for peicing most fabric ecept for whites and very darks. I like to use gray but it isn't a hard and fast rule. I have to use up end of spools from quilting and so I use just about anything.

Gray for special projects though!
peace

I start all my pupils off with a large cone of Grey thread for piecing, explaining how it blends in with so many colours, I haven't used it for quilting as I prefer to use a Natural Cream/Ivory or a lovely subtle tan/goldy colour that I get from Threadart.com it blends perfectly with so many colours.

I use matching colors best/often as I can.but have not had any problem with one color on top and another color on bottom...as long as tension is good. But for generalities...I use a cream/beige/tan color for piece assembly and then matching colors for the top/FMQing. Unless I am using alot of dark colors...then I match closest possible for piece work. I hate having to have to many bobbins "ready to go". My new Singer Quantum...because you have to unthread the machine, rethread to wind bobbin and then rethread machine...I have to wind many....BUT on my good'ol Singer 2010 (now 28 years old!) the bobbin is so perfect....run out? open bobbin cage, press "one stitch" button, and it threads bobbin! I dont' have to unwind, unthread...nothing! Whendone winding the bobbin, i just close the bobbin case, pull the thread out the top and start sewing again! Boy did SInger make a HUGE mistake in design when they took away that feature!

I've found a medium gold or grey/green is very versatile , too.

I use grey alot and feel it is a good neutral.

I use white or beige but now will try gray. That makes so much sense.

I am going to try grey, I generally use off white, or black for both top & bottom.
They are some that I do match color to fabric, and use black or off white on bottom.
I also use a scrap of the material to see what it looks like first.

Yup! it works best.
I use a lot of gray.... lights and darks.
And another one that Coats and Clark used to have (in their cotton covered poly) called "Fawn"... it was sort of grey with a look of dull purple... and perfect for so many fabrics colors !!!!! but alas! can't find it anymore.

I use gray for piecing, except when working on a quilt in distictive colours like bright yellow or red/white, etc. I keep a handful of bobbin spools filled with gray thread and when it comes down to the last 2, I sit and fill up some more. All my tacking is always done in yellow.

Ditto for me..............