Trying to figure out where a gradation might “go astray” can be a real challenge for artists in sn observational, representational effort—let alone when there’s particularly “difficult colors” in play. For example, yellow can be difficult to work with as not only is does it often seem weak in many color mixing scenarios—but due to how we perceive/experience yellow, our estimations of it are often exaggerated in both lightness and chroma.
One way our artists deal with challenging colors or elusive gradations is to use an isolating tool. This is often a uniform surface with two windows capable of showing two target areas simultaneously in an identical context. With our gradation exercises, artists simply move the isolator along the direction of the gradation to see where the disparities occur. You can make this out of any type of material as long as it can stay rigid enough to hold its shape.