Sulky threads, available at Jo-Ann's Fabrics and, I am sure, many other fabric stores come in a huge variety of holographic colors and work beautifully for flashbacks. It can also be furled to make very neat "twist" for ribbing on larger flies. I use it a lot on speys and full-dress stuff, too. It is thinner than Flashabou but easy to deal with.

A huge advantage is the fact the spools are far easier to keep tidy and I need ALL the help I can get for that! They have a "keeper" arrangement on each end of the spool that works very well.
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