The concept of shade and spirit pets are greatly debated amongst the staff and old users. My take is that it was an idea by Patrick - or possibly a test pet - that was either rejected or forgotten.
The lineart had to be greatly changed for the spirit and shade pets to be adapted - and who would want to do the same for over forty pets? Perhaps Patrick decided that it was too tiring for the artists, or that it was too similar and 'unnatural' to fit into Rescreatu's "normal" color schemes? You don't see 'baby' colored pets running around like Neopets, or Bleeding Heart Mirabilis flying around.
Maybe Patrick was unable to get the codes right, or that it was a far too complicated process. Part of me hopes that he will bring back the whole idea.