オノッチ https://twitter.com/onotchi_ さんのこちら http://onoty3d.hatenablog.com/entry/2016/02/06/141002 の記事を元にして、クロマキー合成（クロマキー透過）ではなくクロマキー抽出で肌の色を取り出すシェーダを作ってみた

ChromaKeyOverlay.shader


Shader "Custom/ChromaKeyOverlay" {
	Properties{
		_KeyColor("Key Color", Color) = (0,1,0)
		_HueNear("Hue Near", Range(0, 0.5)) = 0.1
		_SatNear("Saturation Near", Range(0, 0.5)) = 0.1
		_MainTex("Base (RGB) Trans (A)", 2D) = "white" {}
	}

		SubShader{
		Tags{ "Queue" = "Transparent" "IgnoreProjector" = "True" "RenderType" = "Transparent" }

		CGPROGRAM
#pragma surface surf Lambert alpha

		sampler2D _MainTex;
	fixed3 _KeyColor;
	fixed _HueNear;
	fixed _SatNear;

	struct Input {
		float2 uv_MainTex;
	};

	fixed GetHue(fixed3 rgb) {
		fixed hue = 0;
		fixed minValue = min(rgb.r, min(rgb.g, rgb.b));
		fixed maxValue = max(rgb.r, max(rgb.g, rgb.b));
		fixed delta = maxValue - minValue;
		if (delta != 0) {
			if (maxValue == rgb.r) {
				hue = (rgb.g - rgb.b) / delta;
			}
			else if (maxValue == rgb.g) {
				hue = 2.0 + (rgb.b - rgb.r) / delta;
			}
			else {
				hue = 4.0 + (rgb.r - rgb.g) / delta;
			}

			hue /= 6.0;

			if (hue < 0) {
				hue += 1.0;
			}
		}
		return hue;
	}

	fixed GetSaturation(fixed3 rgb) {
		fixed sat = 0;
		fixed minValue = min(rgb.r, min(rgb.g, rgb.b));
		fixed maxValue = max(rgb.r, max(rgb.g, rgb.b));
		sat = (maxValue - minValue) / maxValue;

		return sat;
	}

	void surf(Input IN, inout SurfaceOutput o) {
		fixed4 c = tex2D(_MainTex, IN.uv_MainTex);
		fixed distance = GetHue(c.rgb) - GetHue(_KeyColor);
		fixed distance2 = GetSaturation(_KeyColor)- GetSaturation(c.rgb);
		fixed alpha = 0.0;
		if (distance > 0.5) {
			distance = 1.0 - distance;
		}
		else if (distance < -0.5) {
			distance = 1.0 + distance;
		}
		else {
			distance = abs(distance);
		}

		clip(_HueNear - distance);
		clip(_SatNear - distance2);
		alpha = ((_HueNear - distance)/ _HueNear +(_SatNear - distance2)/ _SatNear)*1.0;
		o.Albedo = c.rgb;
		o.Alpha = alpha;
	}

	ENDCG
	}

		Fallback "Transparent/Diffuse"
}

Webカメラ画像をリアルタイム表示するRenderTextureにこのシェーダーを適用したマテリアルを割り当てると、手など肌の色だけが取り出されて他の部分は透過になる