dolphin/Data/User/Shaders/grayscale.txt

uniform samplerRECT samp0 : register(s0);

void main(out float4 ocol0 : COLOR0, in float2 uv0 : TEXCOORD0)
{
  float4 c0 = texRECT(samp0, uv0).rgba;
  float avg = (c0.r + c0.g + c0.b) / 3.0;
  ocol0 = float4(avg, avg, avg, c0.a);
}