If you go with a 50wt cotton thread, it will be less shiny & will blend right in with the background fabric. Embroidery threads are typically rayon or polyester & those have a sheen to them that draws more attention to itself. Gutermann is a nice inexpensive 100% cotton thread. If you really want a top-notch thread, Aurifil is sold online & in select quilt shops (though usually only in a few dozen colors, not the 260+ colors they sell through online retailers). I'd go with their 50wt. It's a 2-ply and if you get it to match the grey, it really will barely be noticeable even from a foot away.