I don't know if this will work for most of you allowing for my very basic camera skills.
My fly tying mentors insisted that the body colour of the Greenwell darkened as the season went on. This was why they used cobbler's wax.
In the picture from left to right.
Pearsalls Primrose including part that has been passed twice through brown cobbler's wax.
Uni thread Brown Olive.
Uni thread Olive Dun.
I can confirm the changes for I find that a darker body works better later in the season but as to the gradation in colour the jury is out for me.
Still better to err on the safe side.