The reason why? They soak in moisture beneath the wrappings, so creating whitish areas that will eventually (if silk) rotten or otherwise become fragile.
Therefore most fine rods have transparent windings, letting the bamboo expose under the wrapping since the varnish will soak into the thread completely.
The colours blend with the colour of the bamboo to a shade that can be quite unpredictable, especially if the rod is tempered to a dark tone. Below you see what happens on a medium tempered blank:
All windings are Gudebrod Nylon / A (from left to right)
Olive Green 1892, Dark Green 5896, Medium Green 6779,
In the middle a snake ring wrapped with Medium Gray 0720 tipped Black,
Medium Brown 0541, Tan 0290, Gold 0340, Scarlet 0326, Garnet 02406
Note that the Tan is almost invisible as if used White silk, the Gold blends into a medium brown shade and so on. The difference between the two green shades is much more pronounced than if you compare the two spools with untreated thread. Garnet, Olive Green and Dark Green loose most of its character to the dark toned bamboo whiles the influence on Scarlet and Medium Green are much less. The single uninfluenced colour is Black.
Sometimes, while using light shades of Gudebrod Nylon like Tan, White and the like, you will get what appear to be whitish splashes at certain areas. These "white lines" are a nuisance since they don´t blend with the cane. The thread seems to resist the soaking.
Years ago I wrote to Gudebrod Inc. about these phenomena and was asked to send a sample, which I did. I never got a reply so I can´t tell what causes this miscolouring.