Color is distracting. When we look out at the colorful world, it is harder to see value (dark and light). Accurate values are more important than color matching for a drawing. As long as your values are right, you can get away with all sorts of colors.
To help you see values more easily, look through a colored filter. The filter simplifies the world to one color much like looking at a black and white photograph. Look at your subject through the filter, then at your drawing, again through the filter. Do the values match? If not, darken here or there until you see the same values.
The color of the filter makes a big difference. A cyan, yellow, or magenta filter does not block all the colors effectively. Red, green, and blue filters do a great job of masking color. For years I have used red filters. I recommended using them in my drawing books. I noticed that the red filters made blue and green objects too dark. It turns out that each color of filter selectively darkens certain colors. In the diagram at the right, I show how the colors that red, green, or blue filters darken or turn to black. Red filters darken green, cyan, and blue, Green filters darken magenta, red, and orange, and blue filters darken red, orange, and yellow.
Since blue and green figure prominently in most landscape drawings (trees and sky), red filters are not the best choice. I recommend using a green filter. Just use caution when analyzing fall foliage or red rock canyons. I use clear plastic lighting gels used for stage productions. They come in many colors and if you go to a lighting store that stocks them, you can test each one to see how it filters color and value. After testing many gels at a local store I came away with a clear favorite for nature sketching: it is made by Lee Filters in Andover England, Primary Green #139. It does a great job of blocking out distracting colors and the magenta-red distortion does not affect most of the drawing I examine in the landscapes around my home (I think this would be different if I lived in some parts of Australia….). To help your filter last longer, trim it to fit in a slide mount. Trim the plastic 35mm x 38mm the slide mount has a hole that is 24mm x 36mm. Because you can make a lot of filters out of one sheet of lighting gel, make a bunch for your nature drawing friends, school art room, or home school community.