Four answers, and where a white would live if we add one.
Where this landed. No white was added, and no lifting tool. The app keeps the two answers it started with: water, which softens a mark and takes some of it back, and a fresh sheet.
Watercolour has no undo. What it has is water, which lifts some of a mark back off the paper, and body colour, which covers it. Those are different promises: one gives you paper again, the other gives you paint over paint. The mock-ups are at 393pt, the width of the phone, and the engine notes are read off the shaders rather than remembered.
Chinese White is zinc oxide, PW4, semi-opaque, sold as a mixing white. Titanium White is PW6 and is the properly opaque one. White gouache dries more opaque than either, which is why illustrators reach for it rather than for watercolour white. In Japanese painting the same job is done by gofun, ground shell. None of them behave like an eraser: they are paint, and they sit on top, which is visible as a chalky film at any thickness worth using.
No change to anything
The water brush re-wets the sheet and releases what has dried, as much as that pigment will give up. A staining ink barely moves; a mineral one lifts almost completely.
REWET_SKSL multiplies the fixed layer down, gated by the pigment’s staining.What you called alpha reduction
A brush that scales down whatever is on the sheet toward bare paper, without adding anything. The same move as water, but stronger, and not bound by staining. This is what a painter does with a damp brush and a cloth, or a stiff bristle on dry work.
Multiply the live and fixed layers toward zero under the brush. Nothing is deposited.A permanent tile on every stack
PW6, the opaque one. It sits on top of what is under it rather than mixing into it, which is what body colour does. Always in reach, in every palette, next to water.
White has to scatter, and this composite only absorbs, so it needs the field’s spare channel and a covering term before the glaze.One of the five, where it belongs
The same paint, but it appears only in the palettes that would actually carry it, the way a real tin has one white pan among the others.
Same as above. The engine work is the same either way; only where it lives changes.Take the lifting tool first. The need is a mark you do not want, and lifting is the only one of the four that answers it with paper rather than with more paint. It is also the cheapest: the re-wetting pass already multiplies the sheet down, so this is that pass without the staining gate and with its own strength, no new channel and no new colour. A long press on the water tile is where it belongs, because it is the same gesture with more of it.
Then decide about white on its own merits, not as an eraser. White earns its place for highlights on dry work, for the chalky bloom of gouache over a wash, and for tinting a colour lighter without water. If it goes in, put it beside the water rather than in the stacks: the engine work is identical either way, and spending one of five slots in Landscape earths to hold a paint that palette would never carry is the worse trade.