To display only one of the RGB channels of a texture in Spine, you would typically need to manipulate the shader or the rendering process to isolate and display a single channel. Spine itself does not provide a direct option to show only one channel (like red, green, or blue) from a texture. However, you can achieve this effect by customizing the shader used in your rendering pipeline. For example, in a WebGL context, you might modify the shader to output only the red channel by setting the green and blue channels to zero. This would require some programming knowledge to adjust the shader code accordingly.

suanLoBeach I'm a little concerned that you say “like” the spine-unity runtime. I think it would be better if you could clarify which engine you are asking about, as the method of replacing a piece of material varies quite a bit from one game engine to another. (Basically, the Spine Runtimes have almost the same methods for skeleton and animation operations, but the requirements for materials and other rendering-related aspects vary considerably from engine to engine).

Anyway, to answer your question about how to do it in the case of Unity, you can use SkeletonRenderer.CustomMaterialOverride for that purpose. See the following section of the documentation for more information:
https://esotericsoftware.com/spine-unity-rendering#Changing-Materials-Per-Instance