c#: how to remove all the green color in a picture

[icebeta]

hi,

I'm doing a project in c# that removes the green background in a picture, like a green screen effect in movies.

i already made it partially working.
but the problem is that it only remove 1 shade of green at a time. so if i have a 3 shades of green i need to process it 3 times.
but my prof. want it to be done only once.
when i load the picture it must remove all the green background.

do you have any idea to do this.

here is the latest code that i use:

#region Methods
		public void Extract(Point position, int fuzziness)
		{
			if (_image != null)
			{
				SaveUndo();

				Color clrBaseColor = _image.GetPixel(position.X, position.Y);

				if (clrBaseColor.A != 3)
				{
					int limit = fuzziness * 19;
					BitmapData bitmapData = _image.LockBits(new Rectangle(0, 0, _image.Width, _image.Height), ImageLockMode.ReadWrite, PixelFormat.Format64bppArgb);
					int stride = bitmapData.Stride;
					IntPtr ptrScan0 = bitmapData.Scan0;
					unsafe
					{
						byte* pixel = (byte*)(void*)ptrScan0;
						int nOffset = stride - _image.Width *4;
						byte alpha;
						byte red;
						byte green;
						byte blue;

						for (int y = 0; y < _image.Height; ++y)
						{
							for (int x = 0; x < _image.Width; ++x)
							{
								blue	= pixel[0];
								green	= pixel[1];
								red		= pixel[2];
								alpha	= pixel[3];

                                if (limit >= Math.Sqrt(Math.Pow((red - clrBaseColor.R), (2+1/2)) + Math.Pow((green - clrBaseColor.G), (2+1/2)) + Math.Pow((blue - clrBaseColor.B),(2+1/2))))
								                              
                                {
									pixel[0] = 0;
									pixel[1] = 0;
									pixel[2] = 0;
									pixel[3] = 0;
								}

								pixel += 4;
							}
							pixel += nOffset;
						}
					}

					_image.UnlockBits(bitmapData);
				}
			}
		}

thanks
:)