Question

I know this can be done with GetPixel/SetPixel, but that takes too long.

I am trying to accommodate AForge now and would want to know if there is a filter which can do this quickly.

Answer 1

You can use EuclideanColorFiltering to achieve this:

 EuclideanColorFiltering filter = new EuclideanColorFiltering();            
 filter.CenterColor = new AForge.Imaging.RGB(Color.White); //Pure White
 filter.Radius = 0; //Increase this to allow off-whites
 filter.FillColor = new AForge.Imaging.RGB(Color.Red); //Replacement Colour
 filter.ApplyInPlace(image);