Colored and dark papers can save you precious time and give you amazing effects. When I paint/draw/teach plein air I try to nail the impression in under an hour, it is a race against the planet moving. As the sun slowly moves across the landscape you will see new cast shadows, new lights, after 3 hours they all cancel each other out leaving you with muck. So in keeping the time short using the dark paper can be a huge advantage.
The idea is the paper is your dark areas, sketch in the composition and leave the darkest part alone. From there you focus on bringing out the light, driving towards the light, with the last touches happening in the last few minutes before your hour is up! This approach works wonderfully and it feels magical while doing it.
One of the reasons why leaving the paper alone as darks works is because shadows are the absence of light. The darks don’t need details or labor, leaving them alone creates a atmosphere of mystery that is a perfect foil for all the lights you will be drawing. Save time, effort, and create magic by leaving lots of paper alone. Enjoy!
Below are my pastel landscapes all in under an hour. Take note of the dark areas are just paper.