I use polyester. It won't fade. I assume you're talking machine embroidery! For handwork, I don't know.

Hello
If you are doing machine embroidery you need embroidery thread. Polyester embroidery thread is best to use for things that are laundered a lot. It doesn't shrink, fade, stronger than rayon thread. Mettler Poly sheen is one brand. Sulky has rayon that is better for things that are NOT washed alot. 40 wt. is the average size.
Redwork is also called Bluework, one colour designs. It can also have two to three colours to the designs, for filling in some areas of the design. Use the same colour in the bobbin as on top of the machine for something that shows on both sides like a towel. I hope your question was answered.

I'm noy talking about machine embroidery, I'm referring to just the regular embroidery thread, somebody was telling me to soak it in vinegar and water before using to prevent bleeding when laundering.?

hi
I have never done that. I have never heard that.I have never seen it bleed. To get your mind off of it do yourself. Just do a sample yourself to see on a piece of fabric.

Hello,
I just started doing redwork. I am working on a baby quilt for my daughter's baby due in March. The woman who taught me never mentioned bleeding of the thread and she made hand towels for Christmas presents. Also, if my memory serves me correctly, I read where people did not have a problem with the thread bleeding. But, I guess as suggested above, try it and see. I hope this helps a little bit.
Candy