using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Collections;

namespace ProgrammingExercice
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            Bitmap image = new Bitmap("PNG.png");
            Color col = new Color();
            ArrayList wit = new ArrayList();
            pb_image.Image = image;

            int breedte = image.Size.Width;
            int hoogte = image.Size.Height;

            for (int i = 0; i < hoogte; i++)
            {
                for (int j = 0; j < breedte; j++)
                {
                    col = image.GetPixel(j, i);
                    if (col.Equals(Color.White))  
                    {
                        wit.Add(i * 100 + breedte);
                    }
                }
            }

            MessageBox.Show(wit[0].ToString());    //this gives me the error
        }
    }
}

wit.Add(i * 100 + breedte);

if (col == Color.FromArgb(255,255,255))

ArrayList wit = new ArrayList();

Bitmap image = new Bitmap("PNG.png");
pb_image.Image = image;

int breedte = image.Size.Width;
int hoogte = image.Size.Height;
// we're going to be checking against this value a lot
// better do the calculation once
Color checkColor = Color.FromArgb(255,255,255);

// x,y instead of i,j.  Mostly because it makes sense in GetPixel
for (int y = 0; y < hoogte; y++) {
    for (int x = 0; x < breedte; x++) {
        // in contrast, we don't have to assign the results of
        // GetPixel, because we loot at it only once
        if (image.GetPixel(x, y) == checkColor)
            wit.Add(y * 100 + breedte);
        }
    }
}