At some point, it was decided "opaque" paint had no place in "transparent" watercolor. White is considered especially heretical...As though you're not an honest watercolorist if you don't paint around every little white.
It puts me in mind of the Pharisees - a first century religious sect that were fussier than a car wash bill changer.
They felt the 10 Commandments weren't comprehensive enough, so they invented extra rules. Like not eating eggs laid on Saturday because the chicken was performing work on the sabbath...don't want to be an accessory to a sin that egregious do you?
I've already explained watercolor AIN'T transparent and never was. Pigment is pigment.
There are legitimate uses for white or opaque accents. Sure, paper whites look great. They're part of watercolor's unique look. Get 'em if you can. But some stuff is too tiny or complicated to paint around without creating clunky edges or areas nearby.
Using opaque accents as positive, constructive marks - not as correction fluid - should be considered good painting if it produces the desired results.
Sargent used paint opaquely and thickly enough that it cracked over time. Check this one: He used opaque accents for the yellow leaves in the shrubs.
That's what I mean by positive, constructive marks: He meant those, he wasn't fixing boo-boos. The laundry is classic paint-around. What do you bet his picture was the first of this oft-seen watercolor subject?
These Bedouins have thick white highlights on the folds of their garments. Is it so bad? Are those great faces diminished by his methods?
Or how about these gondoliers from his picture of the Bridge of Sighs in Venice? A watercolor Pharisee might think there's enough white paint there to have him stoned. I say it's a great gesture drawing.
Or how about Trevor Chamberlain here: Little deliberate accents like the ship name, cables, gunwhales and such sit just fine with what's otherwise a classic application of watercolor. That mast on the bridge for example: Trying to paint the background around it would have sacrificed the lovely granulation of the color and the area would become a distraction.