/ic/ - Artwork/Critique

Looking for some technical help with pic related. It isn't mine btw.

As you can see on the circled red part, on the character's right arm, the artist shaded the arm in one stroke along the white and yellow zones.

How do I achieve this?
I usually pick each individual color, and manually change it (Darken + hue ajustement) to fit the shading, but here the artist did the shading of both color in one stroke. What's the technique for this?
I tried doing a Multiply layer, but it doesn't work.