There are a lot of opinions on premixing your colors. I have found that at times it is advantageous, but most of the time I don't premix. I'm not talking about mixing tubes of paint, but mixing piles of paint on your palette. You can predetermine the colors that you will have in your painting and premix large quantities of them.
There are pros and cons.
PRO: It is easier to stay consistent with colors and save time. You don't have to start from the beginning each time that you need a color.
CON: It can be too consistent. You start dipping into the paint without consideration of the small nuances and changes in value or temperature.
PRO: It can be easier to maintain proper values through lightening and darkening the premixed colors.
CON: Usually there is a small color shift not just values.
PRO: You don’t have to scramble or go back to square one each time you run out of color. Premixing means you can stay focused on executing your vision, not on trying to re-create a color that you’ve run out of.
CON: Premixed colors can be a crutch of sorts. You may err on the side of sticking with the color you premixed rather than adapting to what you see in front of you. I believe that you can become too complacent in looking for the subtle differences when you premix.
That said, I think when you are an experienced painter you will know whether you can use the premixed color or change it. I have found that it is very helpful when I am painting on location at a time of day when the light will be changing quickly. From experience I can premix some of my colors that I might have in a sunset. I have them ready to go so that I can work quickly, but that doesn't mean that they will be right on. The adjusting that I do will be minor. I have found that this is a time saving effort for me in some circumstances.
Do you have any pros or cons about premixing your colors?