One common method for plein air painting involves laying in an underpainting with the approximate color and value of the scene. When we paint this way, we're juggling two balls at once. It's not easy. If you find your values slipping as you paint, try doing the underpainting in monochrome. I like Burnt Sienna for this. (Studio painters would call this a grisaille.) Establish your darkest value with Burnt Sienna right out of the tube, and then thin it with Turpenoid to create the mid-darks, mid-lights and lights. Next, begin to lay in color, taking care to match the values accurately. The Burnt Sienna gives you guide for this.
Even though the Burnt Sienna will still be wet, don't worry about painting alla prima on top of it. The Burnt Sienna will warm up the painting a bit and create nice greys in your blue sky.
Here's a painting I did this way yesterday. First, the grisaille, followed by the finished painting and finally a greyscale version of the finished painting.
6x8, oil, en plein air