AssetPostprocessor.OnPostprocessTexture(Texture2D)

Description

Add this function to a subclass to get a notification when a texture2D has completed importing just before Unity compresses it.

You can't choose a compression format at this point. If you want to change compression format based on filename or other attributes of the texture, use AssetPostprocessor.OnPreprocessTexture.

However, if you modify the TextureImporter settings in this way, it has no effect on the texture that Unity is currently importing, but it does apply the next time Unity imports this texture. This causes unpredictable results.

using UnityEditor;
using UnityEngine;
using System.Collections;

// Postprocesses all textures that are placed in a folder
// "invert color" to have their colors inverted.
public class InvertColor : AssetPostprocessor
{
    void OnPostprocessTexture(Texture2D texture)
    {
        // Only post process textures if they are in a folder
        // "invert color" or a sub folder of it.
        string lowerCaseAssetPath = assetPath.ToLower();
        if (lowerCaseAssetPath.IndexOf("/invert color/") == -1)
            return;

        for (int m = 0; m < texture.mipmapCount; m++)
        {
            Color[] c = texture.GetPixels(m);

            for (int 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 level, you can modify
        // the pixels in the highest mipmap then use texture.Apply(true);
        // to generate lower mip levels.
    }
}