Photoshop :: Skipio - Color Black And White Antique Prints?

I have many black and white antique prints which I want to colour in photoshop. They are scanned in colour because the paper is old and actually cream and black and white scans of very old paper come out a nasty, dead grey. (Not at all like crisp modern black and white graphics. The etching process of the original means that they are very, very finely detailed.

So far I have been cleaning up the image and then colouring over the top of the black lines. The disadvantage of this is that I lose the sharp blacks. I then go over the important bits with the eraser tool to get the black back but it is very tedious and much is lost. know about opacity and build the colour in opaque layers, and I know the tools to enhance the dark areas such as the dodge tool but these slightly alter the colour and dont restore the blacks lines.

QUESTION: Is there a way to colour in a print and then key out the negative space (white but really cream)and place the cleaned up original black image over the top of the coloured image? This way all the black shading detail would be fresh and clear and the colour will show through.