Extensions for the Unity Texture2D class allowing add/subtract/multiply operations on other textures as well as singe colours.

Texture2DExtensions.cs

using System;
using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;


public static class Texture2DExtensions
{
    public static void Add(this Texture2D tex1, Texture2D tex2)
    {
        if (tex1.width != tex2.width) throw new ArgumentException("Textures must have the same width!");
        if (tex1.height != tex2.height) throw new ArgumentException("Textures must have the same height!");

        for (int i = 0; i < tex1.width; i++)
            for (int j = 0; j < tex1.height; j++)
                tex1.SetPixel(i, j, tex1.GetPixel(i, j) + tex2.GetPixel(i, j));
    }

    public static void Subtract(this Texture2D tex1, Texture2D tex2)
    {
        if (tex1.width != tex2.width) throw new ArgumentException("Textures must have the same width!");
        if (tex1.height != tex2.height) throw new ArgumentException("Textures must have the same height!");

        for (int i = 0; i < tex1.width; i++)
            for (int j = 0; j < tex1.height; j++)
                tex1.SetPixel(i, j, tex1.GetPixel(i, j) - tex2.GetPixel(i, j));
    }

    public static void Multiply(this Texture2D tex1, Texture2D tex2)
    {
        if (tex1.width != tex2.width) throw new ArgumentException("Textures must have the same width!");
        if (tex1.height != tex2.height) throw new ArgumentException("Textures must have the same height!");

        for (int i = 0; i < tex1.width; i++)
            for (int j = 0; j < tex1.height; j++)
                tex1.SetPixel(i, j, tex1.GetPixel(i, j) * tex2.GetPixel(i, j));
    }

    public static void Add(this Texture2D tex1, Color color)
    {
        for (int i = 0; i < tex1.width; i++)
            for (int j = 0; j < tex1.height; j++)
                tex1.SetPixel(i, j, tex1.GetPixel(i, j) + color);
    }

    public static void Subtract(this Texture2D tex1, Color color)
    {
        for (int i = 0; i < tex1.width; i++)
            for (int j = 0; j < tex1.height; j++)
                tex1.SetPixel(i, j, tex1.GetPixel(i, j) - color);
    }

    public static void Multiply(this Texture2D tex1, Color color)
    {
        for (int i = 0; i < tex1.width; i++)
            for (int j = 0; j < tex1.height; j++)
                tex1.SetPixel(i, j, tex1.GetPixel(i, j) * color);
    }


    public static Texture2D Copy(this Texture2D tex1)
    {
        Texture2D tex2 = new Texture2D(tex1.width, tex1.height);
        tex2.SetPixels(tex1.GetPixels());
        return tex2;
    }

    public static void Replace(this Texture2D tex1, Color color)
    {
        for (int i = 0; i < tex1.width; i++)
            for (int j = 0; j < tex1.height; j++)
                tex1.SetPixel(i, j, color);
    }
}