Fenwick rods don't have consistent thread/blank colors. The first photo shows five Fenwick fly rods with different thread and paint colors. The white trim wraps vary from bright white to light yellow. All things considered, Fenwick did pretty well given different thread/paint lots and the rods have aged a few decades.
The second photo shows five brown Gudebrod threads and an FF805 fly rod. The thread on the same rod is not consistent! Also, the 'clear' finish has yellowed more on the signature wrap than the guide wrap. The threads are (L to R) regular #396, NCP #396, NCP #541, NCP #5274, and a second spool of NCP #541. This shows Gudebrod dye batches were not consistent. Unwinding an old bulk spool often shows a deeper color thread inside, again due to aging. No wonder matching new wraps to existing wraps is tough.
With a new build, the owner can pick out whatever color they like. Keep in mind, regular thread darkens when the finish is applied. Regular thread treated with color preservative looks different than unfinished thread. Even NCP thread changes after finishing (darkens slightly).