I sense mismatched terminology which will confuse things for us.

Material - is a grouping of polygons that will be colored by the same algorithm

Shader - a coloring algorithm that is loaded into a material and controls how the polygons of that material will look when rendered

Texture - an image used as data in a shader to drive coloring and lighting effects

Algorithm - a recipe for doing something in a computer - in this (Poser) case the nodes in the shader are used to assemble the algorithm

Node - the boxes connected by wires that represent the built-in functionality of the rendering engine, to be assembled into shaders as you see fit

You used the phrase "Material Layer" which is not a "thing" in the Poser you have, although it is in the upcoming Poser 11. So let's just stop using that phrase right now.

Now - since both the texture and the shader affect the beak, you can manipulate either or both to change how the beak is colored.

Isolating such changes to the beak alone may be a problem, though, if the entire bird is one material. If the beak is a separate material from the rest already, then you are free to make changes in the beak shader or the beak texture that will not alter the other materials. If not, you have the option to use the Grouping tool and change the material of the beak to be different from other materials.

If you decide to manipulate the texture directly (by editing it) you will see the change. If you can't locate the texture, you should use the Texture-Manager to see the full path. Unfortunately you cut off the screen shot of that spot so I can't tell you exactly what to click on. It's the current name of the file next to the Image_Map Image_Source. All I see is "Mac" - click that to bring up the Texture_Manager. Above the browse button, click the pulldown to list all loaded textures. You should be able to find the full path there. Then you know what to edit.

I would actually make a copy of the texture, edit that, and select the altered copy in the Texture_Manager.

If you want to change the color in the shader, there are lots of ways. Literally thousands. You need to say more to get a clean answer to that because there are so many options with different secondary effects that will happen.

The most obvious way to change the color is to edit the Diffuse_Color - it will multiply with the incoming texture, so you can change the ratios of red, green, and blue, but keep the general variations from the texture. Alter the overall brightness by changing Diffuse_Value.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)