Lock Transparency to white lines

Is there anyway to change black line work to white lines? i cant for the life of me figure it out. I've tired playing with the mixer channel, contrast and brightness levels, I've even inverted the colors. Typically when i draw, the lines are black on a white background and then lock the layer transparency, but now that i laid down some color, i want to change a few lines to white. for some odd reason, I can change the line work (black lines) to any other color but white. Thanx a bunch in advance!