I was trying to model a walking stick using a semi-torus and a cylinder. When I placed them close to each other, this is what I got-
When I joined the semi-torus and the cylinder using Ctrl-J, this is what I see-
As you can notice, the color gradient of the cylinder has changed. Based on the order of selection of the semi-torus and the cylinder, either the cylinder color or the torus color is changing.
I can observe this effect even while modelling and not only during rendering. Material for both the objects is the same.
Why is the joining operation affecting the colors? I am attaching the blend file too for your ref.
Walking_stick.blend (534 KB)