TextureMod.cs

using UnityEngine;

public static class TextureMod
{
    private static readonly Material ModMaterial = new Material(Shader.Find("Hidden/TextureMod"));

    public static void Modify(ref Texture2D texture, bool greyscale, bool flip_h, bool flip_v)
    {
        int w = texture.width; int h = texture.height;

        var tempRenderTexture = RenderTexture.GetTemporary(w, h, 0, RenderTextureFormat.ARGB32);

        ModMaterial.SetTexture("_MainTex", texture);
        ModMaterial.SetInt("_Grayscale", greyscale ? 1 : 0);
        ModMaterial.SetInt("_FlipHorizontal", flip_h ? 1 : 0);
        ModMaterial.SetInt("_FlipVertical", flip_v ? 1 : 0);

        Graphics.Blit(texture, tempRenderTexture, ModMaterial);

        RenderTexture.active = tempRenderTexture;
        texture.ReadPixels(new Rect(0, 0, w, h), 0, 0);
        texture.Apply();

        RenderTexture.active = null;
        RenderTexture.ReleaseTemporary(tempRenderTexture);
    }

    public static void ConvertGreyScale(ref Texture2D texture) => Modify(ref texture, true, false, false);
    public static void FlipHorizontally(ref Texture2D texture) => Modify(ref texture, false, true, false);
    public static void FlipVertically(ref Texture2D texture) => Modify(ref texture, false, false, true);
}