Plein-air painting has been a revelation for me in recent years. I only started it in earnest in 2016 but fell in love with it almost straight away. It forces you to decide and act quickly. Architecture, I learnt early in my career, is all about decision making. Good timely decision-making leads to good architecture. In simple terms, is the building to be round or square? is to be introverted or extroverted? is to be clad in brick or plastered? is it be soft or hard? Good decision making as an architect comes about in part as a result, of good clear dialogue with your client. It is essential in fact.
Plein air painting is equally about decision making. You only have a few hours to capture the essence of a scene before the light changes, the weather changes, you get hungry, your fingers get numb. It must be convincing, but it must be simple. Your dialogue with the scene in front of you must be clear and concise. A simple palette of just a few primary colours helps greatly. Of late I particularly prefer, Cobalt Blue, Cadmium Yellow, Yellow Ochre, Burnt Umber, a tiny amount of Alizarin Crimson, and White, nothing else. Most of the greens I am likely to need can be mixed from these, though as a back-up, a bit of Veridian it also useful.
You must be quick particularly with clouds, decisive and sure footed, as best you can. The fact that the open air dries the paint so much faster than in the studio and makes it more responsive helps greatly too; and frees you I find, to just go for it!