Preparing and Mixing your Paints. By Don Finkeldei:  Do your thinking on the palette.  If you  work out your values and colors before applying paint, you'll keep from mucking things up on the canvas.

I can't stress this enough. Very experienced painters can spot mix colors on the palette with a brush and be pretty close to the color, intensity and value they need on the canvas.  Most people can't judge paint that closely.    If you're always needing to re-adjust color on you canvas, consider  preparing  colors/intensities/values on the palette first.  You'll find that  you'll l achieve the following:

Keep your painting from turning into a muddy mess.  The less strokes you have to redo with corrections, the clearer and sharper your painting will be.  Ideally, you should strive to lay down a paint passage ONCE --- and never have to touch it again.   

1. Train yourself  to think about the 4 value planes and 5 types of light BEFORE you start applying paint to the canvas.
     
2. Trains yourself to think about modeling objects with the correct color value and intensity BEFORE you apply paint to canvas.
   
   
3. After a while, you'll automatically start thinking in these terms and pre-mixing before painting won't be necessary (although, many great painters like Scott Christensen still pre-mix objects).

If you haven't read Types of Light and Value Planes, you should do so before continuing.  It'll make understanding this much easier if you do.  You should also read  Paint, Brushes and Mediums, which shows my palette and how I mix grays to adjust intensity and value of primary colors.  I mix with a palette knife, not a brush.  That's what a palette knife is for (also good for sharp edges and accents too).  It's easy to clean off paint on a palette knife... not easy to always clean your brush..

 Here's how I do it (note, This example is for a mid-morning to early afternoon painting.  There are exceptions to every rule):

Keeping in mind the value planes - sky generally being the highest value mass, and uprights being the lowest value mass,  I know that a tree (upright plane) in light (where the sun is shining)  has to be darker as a general rule than any of the rest of the value planes in light. I also know that the sky has to be lighter than any other plane.  I also know that there will be variations of color, value and intensity due to the 5 types of light on the objects I'll be painting (I try to paint objects, not things like  trees, barns or people... all trees are objects, not all objects are trees, it makes it much easier for my mind to grasp).  

knowing all this, I start mixing colors on the palette.  Maybe establishing the sky first.  The Blue or green/blue of sky, Clouds in shadow and clouds in light.   I also know that the sky is warmer  at the horizon than at the zenith,  that clouds in light at the horizon are warmer than clouds in light at zenith, that the underside of clouds are cooler at the horizon compared to the underside of clouds overhead (reflected light overhead to to the ground plane reflecting back up on the underside of overhead clouds).  I will establish several pools of color at the right color,  intensity and value for the sky plane.

Next, I might start mixing my darkest objects, the uprights.  A tree or group of trees in the foreground, for instance.  An object like a tree may have several different types of light to deal with.  Shadow (where the sun doesn't shine), Midtone (transition between shadow and light), light (where the sun shines on the tree), reflected light and accents (both dark and light).  That's the essence of modeling a likeness of a tree.  They're not just a blob of a single paint mixture.   Dealing with all the types of light and shadow will make your object a beautiful  tree.  I'll mix the dark accents first, then the shadow, then the light of the tree, The mid-tone is generally a mixture of the shadow and the light.  I'll mix several colors (due to reflected light), keeping the value the same in each area, warmer, cooler colors, but the values are very close.  I'll end up with 5 or 6 pools of paint that I'll use to model the object called a tree.  When I start applying paint, I don't have to think as much because I worked out all the color/value/intensity aspects for all different types of light as pools of  paint.

From there, it's pretty simple to mix the other areas of the painting, the ground planes, the slanted planes, and knowing that as objects recede, they'll get a little bluer and lighter than object of the same type as in the foreground.

I know that the ground plane areas in light (flat areas near my tree(s) in the foreground MUST BE HIGHER IN VALUE than the lightest areas of the trees (where the sun shines on the trees).  The  ground plane areas in shadow MUST BE HIGHER IN VALUE than the shadow areas of the upright trees.  

Slanted planes like mountains will have more reflected light from the sky moderating their values to somewhere between the uprights and the ground planes.

If there is water, like a lake or stream, I closely analyze it compared to other objects I've mixed.  Water is very simple to paint if you can judge the values in water compared to other values.  A common mistake is to make water too high in value --- and too blue.  Sure, there's a lot of reflection from a sky, but there's also a lot of reflected light from object near the bank and uprights reflected in the water.  There's also the effect of the bottom of the stream or lake affecting the color of the water near you.

Anyway, When I'm done, I'll have multiple areas on my palette with pools of paint,  mixtures for each main object, plane and types of light on them.  I'll possibly re-adjust several times, but when I'm done everything is worked out to my satisfaction ON MY PALETTE.  The thinking is over and the fun of painting begins. I may spend 70 percent of my time on the palette and only 30 percent of my time painting.  I won't have to adjust paint on my canvas much, my painting will be fresher and more painterly and I can capture the scene in a much shorter time, eliminating the changing light.

I'll have time to add accents (usually, just a few) on objects, soften or harden edges and spend time on "finishing" the painting.