A gray scale is a must have tool for any paint box. You can put one in each of your paint kits and have several around the studio like I do. Determining value is the first step in both composition and color mixing. So, as you look at a potential scene it is best to start your design by thinking about the tonal pattern and that it is the organization of light and dark which creates the illusion of space and depth
These scales are simply made from color charts found at the local paint store. The higher end stores have a better selection; charts shown here from Sherwin-Williams. I prefer the warm gray scale which is closer to the gray mix I actually use, made from ivory black and umber.
I use a simple single hole paper punch and create a viewing aperture. Holding the value scale up to your subject, you can isolate the local color and get a more accurate reading.
You can see more precisely the color bias of a hue, the direction it leans on the color wheel either towards warm or cool. And by moving the finder back and forth locates its value and intensity.
I like these charts because they really are disposable, the value scales I made with oil colors I am a little more protective of and keep in the studio. On these Gray Scales I can place dabs of color and do physical comparisons. I use the single color chips which are a mid gray (around a value five on the Munsell scale) also, to really target a color and isolate a color note. Punch a few holes in the isolator and you can relate and compare several color notes in your composition at once. I once made these color isolators out of peoples business cards but I don’t seem to get them anymore plus it was always hard to contact someone after punched with holes and dabbed with paint.
Just a couple notes; a value scale from 0 to 10 is preferred and more common, but you can use anything from an 8 to even a 12. It really doesn't matter because in the aperture, you are looking for the least amount of contrast, you may never actually match the exact value in the isolator but it will give you direction to tweak your color mixture. These charts are missing pure white and pure black; well, you can judge that with out literally having that in your hand. Once you locate the darkest dark and lightest light in your painting you have established your tonal range. Then use this as a tool to help you judge those value relationships and analyze by comparison, degrees of contrast. I have learn so much by using grey scales in the field and observing value in nature. With a sketchbook thumbnail, some color chips and this field study I have more than enough resources and information to re-create the inspiration onto a later larger studio work.
Sunset Study, oil on panel, 5x7
It is in the contrast of light and dark that design happens.
Helen Van Wyk
It is finding the pattern of light and dark that creates interest and strong design and it is in the tonal structure of lights, halftones and shadows that we create depth and describe form. So when it comes down to the components of color, that being hue, value and chroma, collectively referred to as a color note, value is the most difficult and is the most critical.
I recommend that you go for the proper value first, even if it is at the expense of your color. Once you have the proper value you can "inject" the color.