I used Photoshop, but any paint program should work.
Go down in brush size and do some more detailed strokes.
Now the highlight color, a slightly lighter peach. Use the finest brush and squiggle it on wherever there's a left-hand facing edge of the cloud. Again, don't be predictable and regular.
(The best way is to study reference photos, for example this environment textures site has more than 1000 hi res photos in their "Sky" category.)
We need to soften the right-hand side edges of all these squiggles, while trying to keep the left sides as sharp as possible. You can use a small blur brush, or a smear, or simply paint back the base color into the highlight.
Go back and forth, adding more highlight squiggles, softening some parts, bringing the shadow color in with a finer brush too in some places, etc.
On the right is the finished cloud.
Here's a cloud similar to one I saw today, around noon, near the horizon, through my office window. The light is coming not from the side or above, but about 45 degrees or so. This changes where you need to put the highest values; instead of at the edge of each blob of cloud the highest value ends up pretty much centered. So a different technique will be more efficient.
Use a similar ramp to the above, just a bit lighter.
Create a new layer and paint with a sharp edged brush a value slightly lighter and less saturated than the lightest sky color. Darken it a bit at the bottom.
Then add another layer and paint with a fully opaque sharp edged brush the original cloud color, like on the right.
Repeat this cycle until the whole cloud area is filled up. Remember to try to get the edges of each layer a little closer in the beginning, and more widely spaced towards the end (due to the perspective).
Of course instead of the airbrush you could try this with all sharp edged brushes, for a more painterly expressive style.