Have a depth based post shader kind of working, but need help with blending with scene

Below is a falloff post fx shader i'm work on - it works in scene sort of - the far away objects do fall off to blue - however the shader is completely taking over the scene - I'd like it to ideally multiply the color with the existing colors in the scene. How would I do this? (I'm new at post shaders - previously i had built a material shader that does exactly what I want - more or less mapping a gradient to a distance - but I'd like to be able to do it in post, and not have to change every shader in the game)

Shader "Custom/LightFalloffPost"
{
SubShader
{
Tags { "RenderType"="Opague" }

Pass
{

CGPROGRAM
#pragma target 3.0
#pragma vertex vert
#pragma fragment frag
#include "UnityCG.cginc"

uniform sampler2D _CameraDepthTexture; //the depth texture

struct v2f
{
float4 pos : SV_POSITION;
float4 projPos : TEXCOORD1; //Screen position of pos
};

v2f vert(appdata_base v)
{
v2f o;
o.pos = mul(UNITY_MATRIX_MVP, v.vertex);
o.projPos = ComputeScreenPos(o.pos);

return o;
}

half4 frag(v2f i) : COLOR
{

//Grab the depth value from the depth texture
//Linear01Depth restricts this value to [0, 1]
float depth = Linear01Depth (tex2Dproj(_CameraDepthTexture,
UNITY_PROJ_COORD(i.projPos)).r);

half4 c;
c.r = (1-depth)*0.3f;
c.g = (1-depth)*0.6f;
c.b = 1-depth;
c.a = 1-depth;

return c;
}

ENDCG
}
}
FallBack "VertexLit"
}