TextureImporter.SetTextureSettings

Description

Set texture importers settings from TextureImporterSettings class.

using UnityEngine;
using UnityEditor;

// Automatically convert any texture with "SingleChannel"
// in its path into a single channel texture, and set it to use the red color channel.

class SingleChannelPreprocessor : AssetPostprocessor
{
    void OnPreprocessTexture()
    {
        if (assetPath.Contains("SingleChannel"))
        {
            TextureImporter textureImporter  = (TextureImporter)assetImporter;
            textureImporter.textureType = TextureImporterType.SingleChannel;
            
            TextureImporterSettings settings = new TextureImporterSettings();
            textureImporter.ReadTextureSettings(settings);
            settings.singleChannelComponent = TextureImporterSingleChannelComponent.Red;
            textureImporter.SetTextureSettings(settings);
        }
    }
}

Additional resources: TextureImporter.ReadTextureSettings.