How can I change colors in a two color image?

Hi, I have an image of grass photographed close up for use in a packaging project. I have already changed the image from continuous tone to a black & white using hue and saturation for making film positives. However, for client presentation I need to convert the bitmapped image to a green on white image.  I want to change the black to green but leave the white as white. I have tried inverting and using the selection wand without success. I have tried > select > color range and tried to separate the line shapes and dots without success. The bitmapped image has the appearance of a coarse mezzotint. I don't think the color replacement tool is appropriate as it would take forever to do.

Any help really appreciated.