Mourning customs in the Regency Era were less rigid than in the Victorian Era. Excessively strict mourning rules we often encounter in historical novels came into practice after Queen Victoria’s beloved husband died -- she wouldn’t give up her black mourning clothes and turned mourning into a firmly followed rule of propriety. Her subjects used her example to springboard their own mourning customs. Keep in mind that these are not LAWS for mourning. Any display of mourning was done at a family's or person’s discretion. However, there were social norms, that, if not followed, might raise some eyebrows.
I found this: "February 1811 For the Promenade, cloaks in Scarlet merino or grey cloth, black velvet pelisses, lined with grey sarsenet, wrapped plain in tippets; Spanish hats in velvet, or cottage bonnets in black, grey, or scarlet cloth." I can only assume in this instance, the term means the way it was dyed, rather than the color red.
I couldn’t find the official mourning proclamation for Princess Charlotte, but I did find this for the father of Queen Victoria, the Duke of Kent, who died in 1820: