Using Spirit Gum: When using spirit gum, make sure to use a fingertip and, tapping rapidly, "tack" the surface of the glue before pressing the piece down on that area. Usually, a rubber appliance is glued down a section at a time, so that each section gets individual attention.
To blend and seal the edges: If you have some liquid latex, you can use a small piece of rubber sponge to stipple a couple of layers over the edge of the piece to seal it down. Latex-based eyelash adhesive (or surgical adhesive) like Duo is a good substitute. Allow each layer to dry before adding another.
Rubber mask greasepaint: If you can possibly get Kryolan rubber mask greasepaint from Germany, that is one of the best out there and sees much professional use in film and TV. A bit expensive.
You can make your own greasepaint by mixing scrapings of pancake makeup--those densely pressed cakes of fine pigment usually found in a small compact--with a very small amount of castor oil. If you have some flesh colored pancake makeup, you can also, if you like, tint it greenish by adding some strong greenish greasepaint or lining color (from the theatre supply store).
Keep in mind that your home-mixed rubber mask greasepaint needs to contain mainly the castor oil as the vehicle. The presence of mineral oil (petrolatum) in high amounts will be detrimental to the rubber piece. So, whatever you add to the mix, try to keep the mineral oil content down as low as possible.
Rubber mask greasepaint is designed to go on rather heavily. This will allow the whole surface--your skin and the rubber piece--to reflect light the same way (i.e., appear the same).
Once the base RMG is applied, you can shadow and highlight with additional colors. With that thicker layer of castor oil color on the surface of the rubber, you are now in much less danger of the makeup "attacking" the rubber and so small amounts of additional color containing mineral oil may be applied.
Powdering the makeup: After the color is the way you want it, powder liberally with plain talc on a big powder puff and gently press the powder into the makeup, allowing it to absorb a lot of the oil. Then shake the excess powder from the puff, and then carefully pat and remove as much of the powder from the makeup as possible. You may also wish to gently "wash" the residual powder away with a soft rubber sponge and some cool water. The makeup is now "set" and will require, hopefully, only small touchups from time to time.