Hi Marek,
That's an interesting topic. Unfortunately it's a bit too specialized to provide custom Spine shaders, but I'm sure you can easily modify one of the existing shaders with ease.
Basically it just like using a normal color lookup texture, as e.g. used in LUT-based tonemapping, just much simpler (1D instead of 3D).
So you could modify Spine-Skeleton.shader
like this:
_ColorLUT ("Color LUT", 2D) = "black" {}
..
sampler2D _ColorLUT;
..
float4 frag (VertexOutput i) : SV_Target {
fixed4 texcol = tex2D(_MainTex, i.uvAndAlpha.xy); // texcol.rgb containss grayscale color
// this is the only new line in the shader:
texcol.rgb = tex2D(_ColorLUT, float2(texcol.g, 0)); // use green channel of grayscale texture as lookup position
// lines below stay unchanged.
clip(texcol.a * i.uvAndAlpha.a - _Cutoff);
SHADOW_CASTER_FRAGMENT(i)
}