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It describes colour with brightness kept in its own channel. Lightness carries all the tone. The a channel runs green to magenta, and b runs blue to yellow — neither knows anything about brightness. In RGB the two are tangled together, so turning down red also darkens the picture. In Lab you can change one without disturbing the other.

Move the channel the colour you care about sits on. Greens and skin live on a. Skies and warm light live on b. Anything about brightness is Lightness. Inverting a turns summer foliage rust brown while a blue sky barely shifts, because sky is a b-channel colour. The in-app help page shows this with real before-and-after swatches.

The channel probably has little information in it. Tap that channel in the View row to see it alone — if it reads as flat grey, there is nothing there to push, and increasing contrast will mostly amplify noise. Try the other colour channel instead.

The app draws a fast approximation while you are dragging a slider, then replaces it with a more accurate one a fraction of a second after you stop. Strongly saturated colours can shift very slightly when the accurate version lands. Saved images always use the accurate path.

Editing a colour channel pushes a large share of the frame outside what a screen can display. Preserve eases saturation back toward neutral until each colour fits, holding its brightness and hue, which keeps gradation in skies and foliage. Clip cuts at the boundary instead. Because the channels clip separately, hues shift and detail flattens into solid blocks — which is the right answer when you want a graphic, poster-like result.

A layer is the same photograph interpreted differently. Duplicate it, edit the copy's channels, then choose how the two meet. Luminosity takes tone from one layer and colour from the other. Opacity dials back anything too strong — a blend mode that looks ruinous at 100% is often right at 30%.

No. The original stays in your library untouched. Saving writes a new image at the full resolution of the original, not the preview size you were editing.

Adding an image to your library requires permission. Opening a photo does not, because Apple's system picker hands over the one photo you select. If you declined earlier, turn it on in Settings › Privacy & Security › Photos.

No. The app has no network access at all — no analytics, ads, accounts, or tracking, and no requests to any server. See the Privacy Policy.