Hey there, nice job! I absolutely love Sargent's work as well, and any chance to see it in person is always incredible! Don't miss out on his charcoal portrait sketches--quicker (relatively speaking) studies that I think help give insight on how he approached and structured his paintings in color. As for approaching color, I would recommend trying out a limited palette first, a popular one being the Zorn palette --yellow ochre, ivory black, cadmium red light, and titanium white--(and Anders Zorn himself may also be an artist worth studying, especially if you like Sargent). Even if you are approaching this digitally, you can challenge yourself to a limited palette by placing the colors on the canvas and having to color pick from that color wheel rather than the unlimited color wheel in your drawing program. As for resources, Color & Light by James Gurney is the artist's bible to color, and Marco Bucci has a ton of great tutorials on it both on Youtube and here on Proko as well, like this one: https://www.proko.com/s/e2uj But continuing to do studies both from masters, photographs, and real life is the most important. When focusing on color, quick mini paintings that get the main idea for color, with little focus on detail, help get the mileage you need best through repetition and trial and error (I recommend taking a look at plein air painters and groups like Warrior Painters). While learning color theory/science through the resources above, developing an intuitive feeling for it via hands-on practice is key. Hope this helps! :)

You could digitally "glaze" some color over this black and white painting to see if you can learn how color would look with these values. I added a layer, made it a color blending mode, and glazed colors over it to show you what I mean, I hope it helps :)

okay i screwed up and posted the same picture twice because i thought i had already attached it whoops