Let's use Clipping & Airbrush to try and do a gradient fill on skin.
On the previous page, add a ① New Layer on top of the skin's undercoat layer. Turn on ② Clipping for this layer.
Open ① Tool Selection window and select ② Brush Tool.
Next, tap the ①Brush Settings button to open Brush window, and tap ②Airbrush (Normal) . Set the ③ Thickness Slider to 400px (pixels) and tap ①↓ button to close the Brush Setting window.
Next, tap the ① Color button to open the Color window. Use the Color wheel to create a slightly darker skin tone. After you are finished, press the ①↓ button to close the Color window.
If you paint around the border of the undercoat part of the skin with an airbrush, it will create a shading effect. The Current Layer does not protrude from the undercoat because Clipping is set to on.
If you put shadows in all the skin color, it will look like the above.