Moviefreaks suggestion is a good one.
But my take on this job would first mix some color into latex thus coloring all the way through. You can pour in some water based paint (black for this job) into the latex and keep adding and stirring until the color is a solid gray, The white in the latex will turn the paint gray, this is normal as the white will go away as the latex dries. Put the lid on and let the latex sit for a few minuets so air bubbles can escape. The mask will need to have the openings sealed, this can be done by covering the eyes, nose and mouth openings with masking tape. Now with the mask upside down, pour in a little latex inside your mask (which is now your mold). Turn the mask around until a layer of latex coats all the inside of the mold. Now pour out the extra latex back into its bucket and let it drain for a few minuets. Stand the mold upside down to let it dry a few hours until the gray has turned black,( the latex is air cured ) you can now repeat this until you have the desired thickness.
WARNING do not let it sit around for too long between layers, no more than a day at the extreme max, as the latex may not bond to the earlier layer. As it dries out the latex will shrink and pull away from the mold. Dust the inside with talcum powder to stop it sticking to itself and let it sit with the ears pointing down to totally dry for a few days before you try take it out.
I have some long neck Styrofoam wig stands that I put new masks onto to hold their shape during final drying and it is a good way to display them. Do not try to trim its eyes etc until the rubber has totally dried, as it can distort and that will become a permanent aspect to the mask.
You will need Liquid latex, some water based black paint (poster paint can sometimes do) masking tape, talcum powder and a stand to store it on