Relating value (the relative lightness and darkness of things) to color can be a tricky exercise. When color is intense (high in chroma), this becomes even more of a challenge. Since chroma is easily related to brightness, it’s common to associate lightness to it as well, leading to value confusion. This is usually the culprit when we’re having a hard time associating a proper value to a given pastel stick. Grayed tones become much easier to identify. The color is diminished, allowing for the value to be easily distinguished.
It is worth noting that if you’re use to working with oils or other wet media, you may have a distorted mental image of base pigments. Since oils are wet, the base pigment often appears darker, often by as much as two values, and richer in appearance. Think of this like a rock viewed in a stream bed. When wet, it is a jewel, filled with rich lush colors. Once dry it appears dull, lacking the luster that initially drew your attention. This explains why all those rocks I picked up as a child along the streams that ran through the woods of Oregon were nothing more than rocks when I pulled them out of my pockets hoping to impress my mother. To better understand the change in the appearance of pigments when they’re wet or dry, compare burnt sienna straight from an oil paint tube to pure burnt sienna in pastel form (which has not been affected by the addition of white or black). Most manufacturers will indicate pure pigment on their labels or printed color charts. After witnessing the difference many pastel artists decide to work in a higher key, raising the value scale and avoiding overly dark pastel applications. Traditionally black pigment was added to pure base colors to darken them and this produced dead dark tones. Today many manufacturers are combining rich dark pigments in the making of their darker pastels, providing much richer darks.
Training the eye to see value and to not be overwhelmed with color takes time. The best advice I have in arranging pastels by value is to make marks of the color on white, black and middle gray (something close to halfway between white and black), or on a value scale representing no more than nine values between white and black. Then close one eye and squint way down. If the mark nearly disappears, it is close in value. If it stands out as noticeably lighter or darker, you’ll have a better idea of where it resides on the value scale. See, for example, in the illustration above, how—when you squint—the red is close to value 6, the blue is close to value 3 and the yellow is close to value 9.