dolphin/Data/User/Shaders/invert.txt

uniform samplerRECT samp0 : register(s0);

void main(out float4 ocol0 : COLOR0, in float2 uv0 : TEXCOORD0)
{
  float4 c0 = texRECT(samp0, uv0).rgba;
  ocol0 = float4(1.0, 1.0, 1.0, 1.0) - c0;
}