Is there a way to set an alpha color?

I have a bunch of bmp files with a specific solid background color that isn’t intended to be displayed. Is there a way to set that color to be considered as an alpha of 0?

Finally got it. I added this before the Alphatest in one of the built in transparancy shaders (Of course, I also had to add the _AlphaColor and _AlphaColorCutoff params to the top of the shader)

CGPROGRAM
// Upgrade NOTE: excluded shader from OpenGL ES 2.0 because it does not contain a surface program or both vertex and fragment programs.
#pragma exclude_renderers gles
#pragma fragment frag
#include "UnityCG.cginc"

uniform sampler2D _MainTex;
uniform fixed4 _AlphaColor;
uniform fixed _AlphaColorCutoff;

struct v2f {
	float2  uv : TEXCOORD0;
};

float4 frag(v2f i) : COLOR {

	fixed4 texcol = tex2D( _MainTex, i.uv );

 	if((abs(texcol.r - _AlphaColor.r) < _AlphaColorCutoff) && 
 	(abs(texcol.g - _AlphaColor.g) < _AlphaColorCutoff) && 
 	(abs(texcol.b - _AlphaColor.b) < _AlphaColorCutoff))
 	{
		texcol.a = 0;
	}
		
    return texcol;
}

ENDCG

How to make it work on IOS?I suppose that the RGBA texture on iOS is not set up automatically as in android.(how to resolve that?)