There's no right or wrong way to add a color or to blend a color when painting wet on wet. How you move the brush will determine the result. What you get from experience is the predictability of what you'll produce.
In Photo 1 I've blended the top of the cloud into the sky almost totally, leaving the strong white at the bottom. In Photo 2, I've softened the edges of the cloud both top and bottom to create a long, soft cloud.
In Photo 3 I'm brushing out a cloud that didn't work satisfactorily, working the still-wet blue back across the white. In Photo 4, I've just put down a fresh portion of white, and move the brush down, zig-zagging it to create the edge of a cloud.
Painting wet-on-wet is something that gets easier with practice. Start with doing studies, rather than with the intention of doing a finished painting.
