I use 100 wt. silk thread, too. I don't have a lot of colors, just light taupe, dark taupe, grey and black. The spools may cost a lot, but they last forever. I lay a strand of the thread over the applique to see which color blends in the most, and use that color.
There are quilters who do not like to use silk thread, so it's really up to you. If you can find light weight cotton thread, that will work fine - 60 weight is good. With cotton thread I like to match the color to the applique (not the background). Some people even use embroidery floss with good results.
Janet