green mask layer
In Protel, the solder mask layer is a negative layer. That is, any lines or marks on that layer (as seen on the screen) are openings in the actual mask that will be produced from the Gerber files.
The color of the on-screen display for the mask layer are set by hitting the "L" hotkey, and then clicking on the color box next to the name of the mask layer.
When you print the mask layer, all you are going to get is a print of the openings in the layer. To adjust the color of that print, you go to the prefernces setting from pint preview, or Printer Setup>Advanced.
If you mean how do you tell your PCB fabricator to make the mask green, that is done with a note on the Drill Drawing that you supply to the fabricator when you send him the Gerber files.