Yimec The topic allows for quite a lot of different approaches to achieve a similar goal, so please don't consider my list below as complete, I'm sure I forgot some possible solutions.
Outline rendering:
If you want to render only the outline (without the filled attachments on top) there is a bit of a difference to filled-silhouette rendering. That's because the provided spine-unity "outline-only" shaders depend on writing to the depth buffer and using a normal depth-test to correctly discard the inner portions of attachment borders (around each attachment) and only draw the outermost outline border (surrounding the union of all attachments).
So if the outline shall only be visible when behind occlusion objects, you unfortuntely can't modify the depth-test to your liking to e.g. render only if the depth test fails, as then the outline is no longer correct. I'm pretty sure there are clever workarounds possible, like inverting draw order and/or Z scale, but it's not trivial at least.
The situation is entirely different when instead of using the spine-unity outline shaders, you're using a screenspace effect to render outlines as a post-processing effect.
Filled silhouette rendering
Regarding silhouette rendering of a flat color, you could have a look whether these forum threads are helpful:
https://esotericsoftware.com/forum/d/26494-see-through-effect
https://esotericsoftware.com/forum/d/16453-show-a-silhouette-when-hidden
Here it heavily depends whether the fill color must be transparent or can be opaque. If it must be transparent, then you run into the same problem of overlapping attachment pixels darkening, and need to use a render texture as described here. With opaque color, you can save this intermediate rendering step.
In rendering a flat color could be quite easily achieved by writing a shader (or copying and modifying an existing one) to render only when the depth test fails. This can be done without requiring a screen-space effect.
Screen space effects:
There are entirely different solutions available when using screen-space effects to first render your objects to a special "effect-buffer" (be it the stencil buffer or an additional render texture), and later overlay your special effect-color where the buffer is set. This requires both a special shader at the skeleton object to render to the "effect-buffer", and a special post processing shader to evaluate the "effect-buffer" and render the effect color where the effect buffer is set.
There are usually screen space effect asset packs (or tutorials) available, where you then only need to utilize their pipeline shaders at the skeleton to render to their effect-buffer.
