The Chromatic kalimba is a wonderful innovation based on the standard Hugh Tracey diatonic kalimbas such as the Alto and the Treble. Diatonic kalimbas play basically like the white notes on a piano. As long as a song stays in key with no accidentals, you can probably play it on an Alto or Treble kalimba. (An "accidental" is a note that is not in the key signature and requires a "flat," "sharp," or "natural" symbol. For the kalimba, though, this is a foreign concept, as "accidental" usually means "that note isn't on the kalimba.")
If a song changes keys or has accidental notes, and you want to play it on a kalimba, you need a Chromatic kalimba, which has all the notes of the scale, not just the piano's white notes, but all the black ones too - the flats and sharps. Hugh Tracey Chromatic kalimbas are two-sided: they have the sharps and flats on their back sides, and the diatonic keys on the front sides.
"Carol of the Bells" is a beloved holiday song that can at best be performed in an approximate manner on the Alto kalimba - you really need the Chromatic kalimba to get the song just right.