Custom Material / Shader on Spine Animation
I'm not sure why OSX goes nuts trying to read that zip.
This is the shader code for SkeletonFillable.shader:
// - Unlit + no shadow
// - Premultiplied Alpha Blending (One OneMinusSrcAlpha)
// - Double-sided, no depth
Shader "Spine/Skeleton Fillable" {
    Properties {
        _FillColor ("FillColor", Color) = (1,1,1,1)
        _FillPhase ("FillPhase", Range(0, 1)) = 0
        [NoScaleOffset]_MainTex ("MainTex", 2D) = "white" {}
    }
    SubShader {
        Tags { "IgnoreProjector"="True" "Queue"="Transparent" "RenderType"="Transparent" "PreviewType"="Plane" }
        LOD 100
    Blend One OneMinusSrcAlpha
    Cull Off
    ZWrite Off
    Lighting Off
    Pass {
        CGPROGRAM
        #pragma vertex vert
        #pragma fragment frag
        #include "UnityCG.cginc"
        uniform sampler2D _MainTex;
        uniform float4 _FillColor;
        uniform float _FillPhase;
        struct VertexInput {
            float4 vertex : POSITION;
            float2 texcoord0 : TEXCOORD0;
            float4 vertexColor : COLOR;
        };
        struct VertexOutput {
            float4 pos : SV_POSITION;
            float2 uv0 : TEXCOORD0;
            float4 vertexColor : COLOR;
        };
        VertexOutput vert (VertexInput v) {
            VertexOutput o = (VertexOutput)0;
            o.uv0 = v.texcoord0;
            o.vertexColor = v.vertexColor;
            o.pos = mul(UNITY_MATRIX_MVP, v.vertex);
            return o;
        }
        float4 frag (VertexOutput i) : COLOR {
            float4 rawColor = tex2D(_MainTex,i.uv0);
            float finalAlpha = (rawColor.a * i.vertexColor.a);
            float3 finalColor = lerp((rawColor.rgb * i.vertexColor.rgb), (_FillColor.rgb * finalAlpha), _FillPhase);
           return fixed4(finalColor, finalAlpha);
        }
        ENDCG
    }
}
FallBack "Diffuse"
}
Just follow the directions from the thread.
Essentially, just set the _FillColor to Color.white and animate the Material's _FillPhase property.
_FillPhase 0 means it will look normal.
_FillPhase 1 means it will be completely filled with the color assigned to _FillColor.
Animating it from 0 to 1 will make it fade to white.