Customize Zombieland by creating a mod with CustomizeZombieland.ManipulateImage

PumpkinZombieHeads.cs

using System.IO;
using UnityEngine;
using Verse;

namespace Test
{
	public class ZombielandModifier : Mod
	{
		public ZombielandModifier(ModContentPack content) : base(content)
		{
		}
	}

	[StaticConstructorOnStartup]
	public static class Tools
	{
		// Pumpkin.png in Textures with size 128x128
		public static readonly Texture2D pumpkin = LoadTexture("Pumpkin");

		static string GetModRootDirectory() => LoadedModManager.GetMod<ZombielandModifier>().Content.RootDir;

		static Texture2D LoadTexture(string path)
		{
			var fullPath = Path.Combine(GetModRootDirectory(), "Textures", $"{path}.png");
			var tex = new Texture2D(2, 2, TextureFormat.RGBA32, false, true);
			_ = tex.LoadImage(File.ReadAllBytes(fullPath));
			tex.Compress(true);
			tex.wrapMode = TextureWrapMode.Clamp;
			tex.filterMode = FilterMode.Trilinear;
			tex.Apply(true, false);
			return tex;
		}
	}

	[StaticConstructorOnStartup]
	public static class CustomizeZombieland // keep the name 'CustomizeZombieland' so ZL can find your code
	{
		/*
		The arguments are almost self explaining:
		- path is the string from https://github.com/pardeike/Zombieland/blob/master/Textures/Parts.cvs which corresponds to a tile in https://github.com/pardeike/Zombieland/blob/master/Textures/Parts.png
		- Color[] is what you get from Textures2.GetPixels()
		- width, height, rect correspond to the values from Parts.cvs
		- setting noRecolor to true prevents Zombieland to apply zombie colors 
		*/
		
		 // keep the method exactly like this so ZL can find your code
		public static void ManipulateImage(ref string path, ref int width, ref int height, ref Color[] pixels, ref Rect rect, ref bool noRecolor)
		{
			_ = width;
			_ = height;
			_ = rect;

			if (path.StartsWith("Zombie/") == false) return;
			if (path.StartsWith("Zombie/Naked")) return;

			var pumpkinPixels = Tools.pumpkin.GetPixels();
			if (pixels.Length != pumpkinPixels.Length) return;
			pixels = pumpkinPixels;
			noRecolor = true;
		}
	}
}

As someone unfamiliar with RW modding when you say 'creating a mod with with CustomizeZombieland.ManipulateImage' do you mean as a separate dependent on Zombieland with the .cs file within mod/source, or as the packageID, or rather do you mean modifying Zombieland itself? I've been tinkering and I'm a little stuck/confused.

No, the trick here is to avoid a dependency. All it needs is to have your own mod and create that class with that method and Zombieland will detect it at runtime. It then makes a delegate out of it and calls it when needed.