I recently took an online workshop with Susan Abbott, whose paintings I love, especially her watercolors, about Winslow Homer and J. S. Sargent.
Our final homework assignment was to copy three Sargent paintings and a detail from one of his paintings (above). It took me about a month to tackle this assignment, and I still have two more to do.
It's daunting, but I did learn a couple of things: put yellow in your purple; darken green with red; and be freer with brushwork.
1st attempt