PackRGBTool

PackRGB.cs

using System.Collections;
using System.Collections.Generic;
using System.IO;
using UnityEngine;
using UnityEditor;

public class PackToRGB : EditorWindow
{
    private Texture2D[] RGB;

    [MenuItem("Tools/PackRGB")]
    public static void ShowWindow()
    {
        EditorWindow.GetWindow(typeof(PackToRGB));
    }

    private void OnGUI()
    {
        if (RGB == null)
        {
            RGB = new Texture2D[3];
        }

        GUI.enabled = true;

        RGB[0] = EditorGUILayout.ObjectField("Take R From Here: ", RGB[0], typeof(Texture2D), false) as Texture2D;
        RGB[1] = EditorGUILayout.ObjectField("Take G From Here: ", RGB[1], typeof(Texture2D), false) as Texture2D;
        RGB[2] = EditorGUILayout.ObjectField("Take B From Here: ", RGB[2], typeof(Texture2D), false) as Texture2D;


        if (GUILayout.Button("Pack"))
        {
            string outPath = EditorUtility.SaveFilePanel("Where to save", "", "mytexture", "png");

            Texture2D outTex = new Texture2D(RGB[0].width, RGB[0].height, TextureFormat.RGB24, false);
            for (int i = 0; i < 3; ++i)
            {
                string fileURL = AssetDatabase.GetAssetPath(RGB[i]);
                byte[] imgByes = File.ReadAllBytes(fileURL);
                Texture2D readableTex = new Texture2D(1, 1, TextureFormat.ARGB32, false);
                readableTex.LoadImage(imgByes);
                CopyChannel(readableTex, outTex, i);
                outTex.Apply();
            }

            byte[] b = outTex.EncodeToPNG();
            File.WriteAllBytes(outPath, b);
            AssetDatabase.Refresh();
        }
    }

    void CopyChannel(Texture2D src, Texture2D dst, int channel)
    {
        for (int i = 0; i < src.width; ++i)
        {
            for (int j = 0; j < src.height; ++j)
            {
                Color s = src.GetPixel(i, j);
                Color d = dst.GetPixel(i, j);
                d.r = channel == 0 ? s.r : d.r;
                d.g = channel == 1 ? s.g : d.g;
                d.b = channel == 2 ? s.b : d.b;

                dst.SetPixel(i, j, d);
            }
        }
    }
}

If your TAM generator builds bitmaps they won't work and have to be converted into jpegs

Also, to get the right shading, the 6 TAM files [0,1,2,3,4,5] have to be loaded in the RGB slots of this script (via the UI) as r2,g1,b0 & r5,g4,b3