How can I achieve a material that looks like this thin, translucent, frosted flesh, like an axolotl tail?
My approach is very similar to mixing shaders and masks. I use 2 Principled BSDF shaders, because I wanted Subsurface and Roughness on both Layers.
The shader relatively works the same in both EEVEE and Cycles, although EEVEE and its Shaders require the Screen Space Reflection and Refraction settings to be enabled and perhaps tweaked.
outerLayer placed, BSDF node has 2 painted Image Textures and 2 Musgrave Textures to control Roughness and Transmission of the fin and the outer layer of the body.
Subsurface Color controls the tint of this translucent Layer.
innerLayer placed, BSDF node has 1 painted Image Texture, which is connected to the Bump socket. The material tint is also controlled by the Subsurface Color.
Mix Shader Factor
The Mix Shader Factor is controlled by a mix of 2 painted Image Textures and color ramps, which controls the visibility of the veins and the transition between the inner and the outer
Mask Inner Bump
I will post the images of the roughness and the main mask in the future, feel free to use the Blender file below:
The core for Transluence shader was already described.
So just to join the party :)
Three meshes - veins, muscle, skin in one object ... because if I would move veins into a separate object SSS wouldn't be calculated in the muscle shader.
Three materials are just variations of Volumetric-SSS-Transluence-Alpha ...
Note: For more natural result I wanted to use Volume for Muscle for softer blending with Skin, but it was overwritten by Skin Volume :(
With a bit of tweaking Keith Morgan's model