The term "RLM 84" was a post war creation to give a label to an officially unidentified and undesignated color. The color is approximately similar to RAF Sky, although it appears it could vary.
The color was found on the original paint on a He 219 at the Smithsonian NASM. It was not a mutation of 76, but rather a definite color.
Here you can see it sprayed as a squggle pattern over a dark gray (which in itself was not identified).