And sorry I didn't respond sooner!

Yes, I was unclear with the opacity instruction: make sure you have opacity applied to pen pressure when you paint, or if you're using a mouse, use a low opacity setting. The layer itself should be as opaque as possible.

In terms of layer blending with the brushed layers, it depends. In some situations I would use, for instance, an overlay layer or something like it to get the kind of colors and vibrancy I needed. It's really more a matter of experimenting with what works for you than me telling you what works since how I handle the brush may change my outcome versus your own methods.

And in terms of working the mountains, I found it best to be very gentle with the brush to create really soft and nearly transparent forms. If I glopped down too much color it would often look like complete crap and would need to go back at it again with the erase+brush combo.

Feel free to ask more questions if you have them.