One of my favorite subjects

There are so many brands out there, you first choice is cotton or silk
One of the online needlework shops that has a very comprehensive list of fibers is Stitching Bits & Bobs - here is the link to just the fibers she sells:
https://www.stitchingbitsandbobs.com/Fibers.htm
Some of my favorite cottons are DMC Variation, Carrie's Threads (she also has silk now, though haven't gotten any. Yet.), Threadworx, Dragon Floss. Really the only cotton that I really avoid at all cost is Weeks Dye Works - I think it is just awful stuff to work with; brittle, thin, frays terribly and doesn't cover well.
Some of my favorite silks are Gloriana, Thread Gatherer Silk n Colors, and HDF from Vikki Clayton. Also out there is silk & cotton from Dinky Dyes - not bad, but I prefer these other 3 more. Actually, I like them all...
You may have seen the gold-on-black version of Cerque, done by Mark Davis. It's here in Tracy's gallery:
http://www.inkcircles.com/html/Cercles%20Gallery.html - scroll down, it's the 2nd from the bottom. He did that with good old DMC 111. And then Vikki Clayton brought out a silk to match:
http://tinyurl.com/ygq3nb4
Once you pick a color - and make sure you buy enough at one go, because variegated thread other than DMC can vary in dye lots, some by a LOT - the way to work variegated thread is to complete each X as you go, as opposed to doing a pass of ////// and then coming back with the \\\\\\. Completing each X as you go allows the variegation to do it's thing. If you want it to be more muted, then you can do the two-pass method, but it kind of defeats using a variegated.
If you cut your lengths off the skeins so that each end is the same color, you can end and start threads without any trouble. You can also manipulate the colors by strategic stitching, stopping & starting to get your colors to lay the way you want them to. Or you can just go with it. When I did Bouquet for Cheryl, I used 1 strand of Carrie's premium cotton, and juts let the thread do it's thing as I went here & there in the pattern:
http://rifestitch.multiply.com/photos/album/66/2009_Finishes#photo=15
I love Tracy's designs, have several of them in the stash and/or kitted up to do, and did her Book of Ink Circles Stitch ALong last year. Have fun with this design - it's a great piece, and suits itself to infinite color combinations (as you can see scrolling through Tracy's gallery page there!)