There were actually "mourning rules".

Black clothing was worn during a sometimes very long mourning period in the Victorian era and into the Edwardian era although in Edwardian it wasn't as strictly followed. Purple (and shades of purple and I think grey) was worn in a sort of transition stage... coming out of mourning but not quite if you know what I mean. Downton abbey is set in Post-Edwardian era, so mourning conventions weren't as strict then as during Queen Victoria's reign. Queen Victoria actually wore black for the whole of her life after her husband died.