I added some coloured Stylistic Alternates for fancy drop capitals to some of my fonts, just because I can using FontCreator
from High-Logic, but I don't currently have any application that can support them for print or PDF publications. I can only test them in browsers such as Firefox or Vivaldi, which support the technology.
I am not much interested in Emojji, as I don't use a mobile. Web Fonts Test Page
It is not particularly difficult to do for 2-4 colours, and very easy for single coloured glyphs. I wrote a tutorial for creating glyphs with four colours
I don't have much need for it, but those who are more creative than I am may find a use for it.