MissingNo exists in Pokemon Red, Blue, and Yellow, but it looks different in Pokemon Yellow than it does in Pokemon Red/Blue. In Pokemon Red and Blue, MissingNo's color palette is purple and peach, whereas in Pokemon Yellow, MissingNo's color palette is red and yellow. I suggest that Yellow form MissingNo's color palette be used as the Shiny palette for MissingNo in Pixelmon.
Here is an image showing the two blocky forms of MissingNo:
https://imgur.com/a/JNbfyfp
For Shiny MissingNo, the purple can become red, the peach can become yellow, and the black and white keep their colors.