Replacing one color in the texture

Unity Engine
1

I am not experienced enough with the fixed function pipeline. Hopefully someone can help.
At the end of the post is a simple shader which always draws the ‘_MainTexture’ except if it had to draw a pixel with ‘_Color’. In that case it uses ‘_ReplaceTex’.
Is it possible to write such a shader without cg?

Shader "Experiment/Replace Colored Area" {
	Properties {
		_Color ("Replace Color", Color) = (1,1,1,1)
		_MainTex ("Main Texture", 2D) = "white" {}
		_ReplaceTex ("Replace Texture", 2D) = "white" {}
	}
	SubShader {
		Pass {

CGPROGRAM
#pragma vertex vert
#pragma fragment frag

#include "UnityCG.cginc"

float4 _MainTex_ST;
float4 _Color;
sampler2D _MainTex;
sampler2D _ReplaceTex;

struct v2f {
	float4 position: POSITION;
	float2 uv: TEXCOORD0;
};

v2f vert (appdata_base i) {
    v2f o;
    o.position = mul (glstate.matrix.mvp, i.vertex);
    o.uv = TRANSFORM_TEX (i.texcoord, _MainTex);
    return (o);
}

float4 frag (v2f i): COLOR {
	float4 texColor = tex2D ( _MainTex, i.uv);
	if (texColor.xyz == _Color.xyz) {
		texColor = tex2D ( _ReplaceTex, i.uv);
	}
	return (texColor);
}
ENDCG

		}
	}
}
2

There are no conditionals in the fixed function pipeline, except for alpha and Z testing. Available fixed function texture operations are listed here: Unity - Manual: ShaderLab: legacy texture combining

3

Thanks for your answer. I expected that it would not be easy. So I will try to use ‘AlphaTest Equal’.

4

Is there some reason you can’t do this with an alpha channel?

5

It is possible that there will be more than 256 different colors or alpha values. But I could split it up for that case. The first tests were successful and the shader was very simple to implement.

Shader "Experiment/Replace Alpha Area" {

	// Idea:
	// Always draw '_MainTex'.
	// In the case that '_Alpha' and the alpha value in
	// '_ReplaceTexWithAlphaLookup'
	// are the same, draw the rgb colors of
	// '_ReplaceTexWithAlphaLookup' without
	// it's alpha component.

	Properties {
		_Alpha ("Replace Alpha", Range (0.0, 1.0)) = 0.0
		_MainTexture ("Main Texture", 2D) = "white" {}
		_ReplaceTextureWithAlphaLookup ("Replace Texture with Alpha Lookup", 2D) = "white" {}
	}
	SubShader {
		Pass {
			SetTexture [_MainTexture] { combine texture }
		}
		Pass {
			AlphaTest Equal [_Alpha]
			SetTexture [_ReplaceTextureWithAlphaLookup] {combine texture}
		}
	}
}