After you looked at Strack, I looked at Kunz and if, in both cases, we are looking at pearls more than 120 -150 years old, many rivers in Europe produced fresh water pearls as well as Britain. These countries included Lithuania, and Sweden, Austria, and other countries with rivers running from the alps. Most of the mainland European ones were described as brownish being the most common color.
Kunz specifically said among American freshwaters, the brown ones are worth less because their luster is not as great. It seems the best colors and luster came from the Mississippi and from New York on south to Florida, there were more browns. The pearl rush in America was virtually over by the turn of the 20th century.
Strack also mentions that at the turn of the century,many dealers could still tell the origins of the American freshwater pearls by looking at them. Too bad they didn't have color photography! We seem to have lost a lot of that knowledge, but a few people, like Norm, know more than average.