This picture is from the English ladies magazine La Belle Assemble, dated to 1827. The name of the magazine of course wants to invoke thoughts of the Parisian fashion and not of London - where it was published.
This picture shows to ladies wearing evening dresses, one labeled 'ball dress' and the other 'opera dress'. This does not imply that you were supposed to wear a special dress when you were going to the opera, though, just that the cloak the woman is wearing is an opera-cloak. The dress worn underneath is very much an evening-dress in a model similar to the other one on the picture.
At the beginning of the 1820's the regency silhouette was still very much present in the fashion with a waistline just under the bust and a narrow skirt. But by now we are seeing a very decisive move away from that towards the more typical 19th century style dress. The waist has moved down to where the waist normally can be found on ordinary women, at the same time as the skirt is getting wider, though not as a wide it would become in later decades.
The return of the proper waist also meant the return of the proper corset. Very few women could ever actually achieve a waist of the kind seen in this picture - this is after all a drawing of an ideal, but that didn't stop people from trying their best to do it.
The light colours of the dresses could be thought of as a heritage from the light colours of regency fashion - but in reality it is not. By now the day-wear was much darker, it just was a fashion that evening wear at this stage should be in lighter colours - and it was even quite possible to have them in white (this was before it was customary to reserve this colour to the blushing brides).
The difference between these two dresses is without a doubt that one of them was meant to see complete with out door-apparel while the other one was meant to be seen without. That is why the opera cloak was a specially made article of clothing, it was not just any ordinary coat - thought without a doubt people who were a little less well off still had to be satisfied with that - and the hat was a show-piece too. The ball-gown was just accompanied by a shawl, so thin that it is hardly noticeable.