Since Pearsall discontinued producing their Gossamer Silk thread many of us have been in search of a replacement silk thread. I have chosen to use the YLI silk thread colors. Although YLI silk thread is slightly thicker in diameter than Pearsall's Gossamer silk thread I have not found that to be an issue for tying wingless wet fly patterns. The YLI colors do not match Pearsall's Gossamer silk thread colors, as you will see in my photos below. But, I doubt that the trout will care, which is what matters to us, fly fishermen.
YLI silk thread comes on 200-meter (656 feet) spools. The color numbers are marked on each spool which is a nice plus. Pearsall's Gossamer didn't put the color number on their spools which always made it more difficult to determine the exact color. The YLI's spool lid pops up slightly so you can secure the loose thread ends and avoid the thread from unwinding and making a mess. The YLI spools are a lot larger in size which makes them more difficult to insert into the normal fly-tying bobbins. I have not found that an issue and I often tie my wingless wet fly patterns with a good wax and without the need for a bobbin. The price per spool for YLI silk thread is around $6.25, which is a great price compared to what you will pay for a spool of Pearsall's Gossamer silk thread on eBay if you can even find the color.
I do feel Pearsall's Gossamer Silk thread is the standard especially if you want to tie the old historical fly patterns. For most of my fly tying, I am trying to tie wingless wet fly patterns to imitate the aquatic insects that I see when fly fishing. So I am not concerned with being historically correct, nor do I refer to my patterns with the old historical pattern names. There are also a lot of other options for silk thread too. Find the one you like and I am sure the trout will agree.
Here is an example of tying size #14 wingless wet flies with YLI silk thread.