How to Change Pixel Color of Image in C#

Copied from the Site: http://stackoverflow.com/questions/17208254/how-to-change-pixel-color-of-an-image-in-c-net

var originalColor = scrBitmap.GetPixel(i, j);
if (originalColor.R == 0 && originalColor.G == 0 && originalColor.B == 0)
    newBitmap.SetPixel(i, j, Color.FromArgb(originalColor.A, Color.Red));

Now you'll see that it works but it takes a very long time to process each image: GetPixel andSetPixel are pretty slow (primary because they check and calculate everything for each call). It's much better to handle bitmap data directly. If you know the image format in advance (32 bit-per-pixel to 8 bit-per-pixel) then you can do it much much faster with LockBits:

The Bitmap class provides the LockBits and corresponding UnlockBits methods which enable you to fix a portion of the bitmap pixel data array in memory, access it directly and finally replace the bits in the bitmap with the modified data. LockBits returns a BitmapData class that describes the layout and position of the data in the locked array.

static unsafe Bitmap ReplaceColor(Bitmap source,
                                  Color toReplace,
                                  Color replacement)
{
  const int pixelSize = 4; // 32 bits per pixel

  Bitmap target = new Bitmap(
    source.Width,
    source.Height,
    PixelFormat.Format32bppArgb);

  BitmapData sourceData = null, targetData = null;

  try
  {
    sourceData = source.LockBits(
      new Rectangle(0, 0, source.Width, source.Height),
      ImageLockMode.ReadOnly, PixelFormat.Format32bppArgb);

    targetData = target.LockBits(
      new Rectangle(0, 0, target.Width, target.Height),
      ImageLockMode.WriteOnly, PixelFormat.Format32bppArgb);

    for (int y = 0; y < source.Height; ++y)
    {
      byte* sourceRow = (byte*)sourceData.Scan0 + (y * sourceData.Stride);
      byte* targetRow = (byte*)targetData.Scan0 + (y * targetData.Stride);

      for (int x = 0; x < source.Width; ++x)
      {
        byte b = sourceRow[x * pixelSize + 0];
        byte g = sourceRow[x * pixelSize + 1];
        byte r = sourceRow[x * pixelSize + 2];
        byte a = sourceRow[x * pixelSize + 3];

        if (toReplace.R == r && toReplace.G == g && toReplace.B == b)
        {
          r = replacement.R;
          g = replacement.G;
          b = replacement.B;
        }

        targetRow[x * pixelSize + 0] = b;
        targetRow[x * pixelSize + 1] = g;
        targetRow[x * pixelSize + 2] = r;
        targetRow[x * pixelSize + 3] = a;
      }
    }
  }
  finally
  {
    if (sourceData != null)
      source.UnlockBits(sourceData);

    if (targetData != null)
      target.UnlockBits(targetData);
  }

  return target;
}

Note: To run the unsafe code in C#, you have to do: Open the properties for the project, go to the Build tab and check the Allow unsafe codecheckbox.