'Harmony cel' is a term used for cels where the 
bulk of the image that is usually a cel (mostly 
characters, mecha, etc) and the usual background 
is painted on background stock.  Usually these 
come with a normal 'acetate' cel with just the 
black lines to give the image definition.  Most 
harmony cels I've seen have been watercolor on 
background stock.  

There was a somewhat recent show me of harmony 
cels and many people have examples in their 
galleries if you want to do some digging.

Hope this helps!  Lovely example you have!

Keys