Create a second image located at exactly the same position as the first, with the lines that you want, and with AlphaData set to 0 where the underlying image should show through, and 1 where the overlay should show through. The second image would have to be truecolor (RGB) to use different colors than are in the original image. Note that if you do this then you will need to use the OpenGL renderer as it is the only one that supports transparency.
Alternately after drawing your base image, use line() specifying RGB colors to draw on the grid.
It is not possible to "exempt" particular values from the colormap: if you are using pseudocolor then every color in the image must appear somewhere in the colormap.
