This style of colouring resembles traditional painting because the brush strokes are still visible. It doesn't matter that much if the line art is kept or not but I prefer to thin or paint it out to give more of a painting feel ^_^ I suppose it is possible to paint with a mouse but there's less control over brush pressure and stroke placement compared to a tablet/graphic pen. So I highly recommend using a tablet for digital painting. This page has a lot of images on so may take a while to load.

NOTE: If you have no experience with photoshop or layers I recommend reading the cell - photoshop tutorial first or playing around in photoshop first. 

To give a paint brush kind of stroke the air brushes should be used but double clicked so this menu appears and adjusted to be hard. If they're soft then they're fuzzy round the edges and not great for painting with.

There are many ways to colour and form clouds in pictures. It all depends on the mood you want to give with the picture. This picture is tranquil so I made softly rolling clouds with pastel blues that fade in the sky. I used the same layer all this time as I steadily built depth into the clouds. When I wanted to add deeper shadows, I use a more purple tinted blue instead of a really dark blue so the picture isn't darkened but shadows are still given. I use the colour dodge tool for some patches of white to really bring out depth through contrast. Once I'm satisfied I add very faint streaks of yellow to give warmth from the sun. I dabbed a few faint circular brushes on to give a camera lens effect as well as a few colour dodge dabs for sparkles of light.

I made these clouds up because I'm used to drawing them and have observed them enough to imagine them but if stuck for cloud shapes, reference can always be used. Just do an image search ^_^