Value Viewer is a very useful app that I frequently use to check whether my composition is working from a lights and darks point of view. It costs about $8 and you can use it to see your work as NOTAN or a grayscale that you can darken or lighten. If the NOTAN shows a strong design, the painting should work too. Here's an example. If it doesn't show a strong design, and has the lights and darks scattered around, for me that means it needs some rethinking. It's taken me many years to really get that!
Plein air painting in the California desert is a lot of fun! In the spring our WOBA (Whistler Out-of-Bounds) foursome (Lianne Gulka, Susie Cipolla, Andrea Moore and I) painted in 30º weather .. the acrylics dry out so quickly we now understand why so many southern artists prefer oils! Gotta watch out for tarantulas and rattlers, but otherwise it was a great experience with a whole new palette. Might take a few more visits to master those warm subtle colours!