color replacement:  Is there a way to make the background of a line drawing transparent so it can be superimposed onto a photo?

I asked a similar but different question a week ago but I never got an answer nor did I respond.  I found the answer to that question on Lynda.com.  It is easy enough to make allayer transparent using lasso type tools but with many line drawings there are so many pixels of one color isolated from each other that it might take several weeks to Magic wand each one.  However if each pixel could be replaced by a transparent pixel the image on a layer would be visible through the overlaid drawing.  I've had no problem with text messages but line drawings are a problem.  I once used Corel Draw and it seemed to be able to deal with this issue so I thought maybe Photoshop would have an easy solution.