AssetPostprocessor.OnPostprocessTexture  

// Postprocesses all textures that are placed in a folder
// "invert color" to have their colors inverted.

class InvertColor extends AssetPostprocessor {

// Use this for initialization
function OnPostprocessTexture (texture : Texture2D) {

// Only post process textures if they are in a folder
// "invert color" or a sub folder of it.
// var lowerCaseAssetPath = assetPath.ToLower();
// if (lowerCaseAssetPath.IndexOf ("/invert color/") == -1)
// return;

for (var m=0;m<texture.mipmapCount;m++)
{
var c : Color[] = texture.GetPixels(m);
for (var i=0;i<c.Length;i++)
{
c[i].r = 1 - c[i].r;
c[i].g = 1 - c[i].g;
c[i].b = 1 - c[i].b;
}
texture.SetPixels(c, m);
}

// Instead of setting pixels for each mip map levels, you can also
// modify only the pixels in the highest mip level. And then simply use
// texture.Apply(true); to generate lower mip levels.
}
}