In watercolor, white is the paper. It must be protected from the start. Sometimes you can get paint to come back up, but only certain colors. It’s always a challenge to keep the white areas clean. Sometimes I use a latex mask, occasionally tape, but that creates another set of issues because it can rip the paper if you aren’t careful.
In oil, white is paint. Six different ones. With slightly different working qualities. Then it gets mixed into the colors, or painted on the surface. It’s not what I am accustomed to thinking about. It’s one of the reasons for the media shift. I was building layers in my watercolor map paintings and wanted to put white on the top layers. It’s useful for defining roads. I didn’t always know where things would go at the start, so I didn’t have the ability to protect the whites. I found myself compromising by adding white gouache or painting back in with watercolor ground.
Now I have a selection of different whites to learn about. And I can add it over the top of colors like I did in the painting above.