Me : Here I added with a dark brush some shading in the lower part of the cloud.You can see the settings so it wont be complicated to see the soft blended shape .Don't forget you can download the file if you need to ( file at first page).

Fun-chan:I got an airbrush tool and would use the eyedropper tool to get the color in between the light and the dark. Then go back and forth between the dark, light, and medium colors until it looked smooth and blended.

*Note:Picking colors with the eyedroper ir very helpfull to get more tones into the clouds.