Methods and Systems

In "Side Street in Rome" I approached the painting with a design that conveyed a sense of light and shadow. The sense of contrast is kept fairly subtle, the painting stays predominately in a middle value range. I massed in the large pattern first to establish the design and to remind myself to stay within a set value range for the lights and another for the areas of the image that were in shadow and indirect light.
I tend to mix on my palette with value in mind. The middle of my palette will have the puddles of color that are in the mid value range, the darks are mixed to the left and the lights to the right. The value puddles mimic the arrangement of colors on my palette. My palette is arranged from warm to cool from right to left and my white is on the right hand side, I'm right handed and the placement of white on the right makes sense to me. After years of working this way it has become second nature.
To get my point across when teaching students about value I compare light and shadow to Mafia families, the lights are the Colombos and the shadows are the Gambinos, if everybody stays where they belong in the picture nobody has to get rubbed out. So if you are painting an area in your image that is part of the shadow, not being lit directly, you load your brush from the middle and dark puddles on your palette.
I don't premix a range of values and colors, I can see it is a helpful method and it does help yield consistent results and the consistency is part of what I don't like about paintings of the premixed palette approach. There tend to be the same color and value choices regardless of the image being depicted and often times the darks never seem rich enough.
There is also a tendency to use the premixed color because the value corresponds to what is seen even if the color doesn't. There is a lot to be learned from various methods and systems and control is a great asset, until it controls an artist's willingness to take chances and respond to what a particular painting calls for. Method and system driven art tend to be safe choices offering few surprises along the way, there's a bit of a guaranteed look. There are key principles in approaching a painting or drawing, training and developing a reliable set of skills and working knowledge is crucial, just don't let the technical aspect of creating hamper your range of choices.
A work of art is the trace of a magnificent struggle. (Robert Henri)