Here's an online source.) 2) on the bottom is Gutermann #R753, which is heavier weight silk and is known as buttonhole twist. I use this less, but it does come in handy for making thread chains, like for swing tacks on linings. (Here's another online source which carries both weights in all colors.)
The regular-weight thread on the blue spool is the kind of thread used for most handstitching in tailoring, like pad stitching, basting, and catch-stitching hems and seam allowances. The heavier twist is used for the exquisite handworked buttonholes in expert tailoring, like these. The reason for using silk? Like silk fabric, silk thread is smooth and strong. It slides easily through fabric and interfacing and doesn't show press marks when steamed. Run it through beeswax for extra strength and less tangling. (Casey did a great tutorial yesterday on this; definitely go check it out.)
As for color: as a general rule, you want to match your silk thread to your fashion fabric, but don't worry too much about an exact match. With pad stitching, your stitches won't show on the outside of the garment. For hems and other hand stitches, it's nice to have a thread color that blends with your fashion fabric, but not essential. I would say that matching color is less important than getting silk if you have to choose.
Need a silk substitute? In Cool Couture, Kenneth D. King recommends rayon embroidery thread as a readily-available and affordable option. The brand that most retailers carry seems to be Sulky.
I'm pretty sure this covers it for now. Let me know if you have more questions!
P.S. I thought you might enjoy Pip's participation in my silk thread photoshoot this morning. First, she posed in silhouette behind the curtain.