I use Hainsworth Doeskin woven wool cloth for my button bushings. Most of my clients opt for the default black option:
However I also stock it in cream, and because it is 100% natural wool, it is possible to custom-dye it many different colours. For example, No. 13 has a darker and more muted shade of red than the garish scarlet you sometimes see:
No. 7 has leaf-green bushings, which I think work particularly well with the glass buttons:
No. 3’s bushings appear black in photos, but in reality they are actually a very dark green.