When you apply a beard on an elf you're able to hue it with ML colors such as glacial or glowing blaze. It's the same method as doing it on humans.
Just apply the hair dye on your hair alone when you have no beard, then apply the beard cream. Be careful on the style of beard you choose because there's no going back unless you spend another truckload of gold on another beard cream.
When the beard grows in, it will adopt the color of your current hair. At that point, if you wanted to do double hued hair/beard you would have to use an ML hair dye again to change just your hair.
However, if you want to ever get rid of your neon beard or recolor it, the only options you have are the hair dye bottles from the provisioners. they cost 37gp each and have a limited selection of colors. The white color from them quite nearly matches the pure white ML hair dye though.
I'd save it for a gargoyle if I were you though