So we've established that silk thread is superior to nylon. Now, what about the thread color? Well, one of the things a rod maker has to know (& let their client know) is that silk thread changes color when varnish is applied to it. Sometimes this color change can be dramatic. What looked like a red thread on the spool might turn to brown under the varnish; yellow thread might just disappear altogether against the bamboo; a blue or green thread may look more like black against the black graphite rod shaft. So it's extremely important that the rod maker know this before wrapping the rod. Color samples might be in order here.
The photo below demonstrates how silk thread changes color under varnish. Both guide feet are wrapped in the same thread, but you can see how much the color has changed.
This change of color can be prevented with a treatment called 'color preserver'. The preserver locks in the pigments in the thread allowing the color of the thread to remain the same under the varnish as it was on the spool. So why not use color preserver all the time? Well, if you treat the wraps with the preserver, it takes away any transparency. You won't see the guide feet & this is a look that some folks want because it shows that real silk & real varnish was used on their rod - quality, traditional methods of craftsmanship went into making their fly rod.
Choosing the exact wrap color you want on your rod can take time & patience. You'll have to experiment a little & plan ahead when looking at spools of thread. In the end though, I don't think anything beats the look of a rod wrapped in fine silk thread & coated with varnish. Those classic good looks never go out of style.