Using Grays in your Oil Painting.  By Don Finkeldei:  How to achieve harmony, balance and reserve in your oil painting by pre-mixing grays from the primary colors.

I use a limited palette (cadmium red light, cadmium yellow medium, cadmium lemon yellow, ultramarine blue and a little Alizarin and viridian).  Rather than adjusting the tube colors with complementary colors and white to decrease intensity and adjust value, I use grays that I pre-mix to moderate the tube colors.  I mix a very dark gray (almost black) by using 2.5 parts of ultramarine blue, 1 part cadmium red light and 1 part cadmium yellow medium (I use Utrecht paints, other brands might require different parts). You can also use orange since that's the same as equal parts of yellow and red.  This combination makes a very dark neutral gray ( that I’ll use to mix the darkest shadow colors for the painting).   I mix a lighter cool gray by adding white and blue to the dark gray and a lighter warm gray by adding white, yellow and red (a khaki gray color).  Sometimes I mix three or more value ranges (darker to lighter) of cool and warm grays, especially if a painting will contain primarily shadow areas where I want fine control over the color intensity and values in the shadows.

There are three properties of any paint you use.  They are hue (color), value (lightness or darkness as viewed with no chroma - grays) and chroma (color intensity).  A pure gray has no chroma. Pure primary colors (cadmium red, Cadmium yellow or ultramarine blue)  have maximum chroma.  Value is the lightness or darkness of the paint as seen without color (gray scale - what it would be if  on a scale of black to white).  Using  neutral grays to adjust  chroma  and value  without modifying the hue is obviously an advantage with this technique and will give you much greater control in mixing paints.  As an example, lets say you mix pure red and blue together to get a purple color (hue).  You can then use the neutral grays (of the right value you need) mixed with the purple to attain the correct chroma (color intensity) and not have to worry about changing the color.  An intense chroma of purple ( in this case ) is lessened by  graying the purple (making the chroma less intense) and the value (lightness/darkness) is modified by either using a darker or lighter  value of neutral gray (lighter values have more white in the gray, darker values have less white in the gray).

I almost never use pure primary tube colors on the canvas directly.  The reason is that a scene in nature almost never contains pure primary colors either.  Another reason:  using grays to moderate the primaries allows you to build a reserve into your painting and creates harmony.  You will want to de-emphasize and subdue certain areas of your painting allowing your focal objects and areas to take precedence.   If you don’t construct a painting with reserve in mind, you won’t be able to achieve an effective focal point with that extra punch you need.  Use the grays to adjust your primary colors to the right intensity and value and you’ll find that your learning curve and accuracy will increase very quickly.

Using an ample supply of pre-mixed grays of varying value and temperature (cool (more blue in the neutral gray),  and warm (more yellow and red in the neutral gray)) will allow you to arrive at just the right mixture much quicker than trying to mix each brushstroke without them.  Your plein air painting will also be completed quicker.  That’s important when lighting conditions and clouds vary rapidly throughout the day.

I can't stress enough the importance of being able to mix grays to moderate tube paints. Also, notice that my palette background is a mid value perfectly neutral gray.  That's important because the palette background color will skew your perception of the color you mix.

Don't buy a lot of tube colors thinking you can use them as they are.   It's a bad crutch and will not  teach you the important nuances of how to arrive at the exact hue, value and chroma you need.  Learn how to mix EXACTLY what you need because paint out of  tubes almost never are "exactly what you need".  All is needed are the primary colors and a variety of grays.  from that, any color value and hue can be achieved.  I can arrive at yellow ochre, cobalt blue, orange, brown, purple, or whatever color/value/chroma that I want from the three primary colors and grays.  It's also much less expensive than buying a huge array of tube paints.

Don't use different manufactures of paint.  Be Consistent.  Don't use barium reds or off colors of the primaries.  Stick with CADMIUM RED, CADMIUM YELLOW and ULTRAMARINE BLUE made by the same manufacturer.  They are the closest colors to the primaries.  The use of a cooler yellow like CADMIUM LEMON YELLOW ( a cooler yellow than Cadmium Yellow) and transparent colors like VIRIDIAN and ALIZARON (good for shadow areas) can be a valuable addition to your palette (I show them on my palette below).  Don't buy student grade paints.  They don't go as far as the professional grades and are very weak in pigment.  Learn the proportions needed to mix a flat neutral gray with your manufacturers paints (most important - grays are moderators for value and chroma).