Currently in Lightroom there appear to be only 5 colour choices for labelling photos. I group photos using colour labels, but I frequently have more than 5 groups (eg. panoramas), and I find the star systems not appropriate for that purpose. Is there a way to add additional colour labels to the existing group?
There are essentially those five colors. BUT you can add different names to them; to see examples, look at Metadata>Color Label Set>Edit... You could have a set for use with say panoramas, one for HDRs, etc. But an image can only have ONE label at a time.
A MUCH more flexible and scalable system is to use keywords for the same info, like Photo>Panorama, and Photo>HDR, and Photo>Bracket, etc. You won't see a color in the grid view, but the Keyword List makes it easy to get to whatever you want. And photos can have any number of keywords, so you can have both "To Do" as a keyword and "For Print" or whatever.