Odd that it would say not to thin it, although I could see having issues trying to thin old shellac.
Shellac usually comes in a 3lb. cut and is generally thinned for everything from coating floors to doing a french polish.
For infrequent use, it is best to just buy the flakes and mix in small batches fresh as needed.
If you want to go nuts you can use aniline dyes or other methods to further color, but need to be careful on application as dyed shellac can leave streaks if you try and go back over your work. Best to keep moving forward with dyes.
Good luck with the shellac share. I still prefer it for my floors.
-Justus
Mpls, MN