A rich and diverse landscape of brushstrokes can add a great deal of interest and aesthetic quality to a painting. However, one of the factors that painters can sometimes under-appreciate is the influence of paint stroke direction on its perceived value/color.
Understand that a paint stroke is perceived via an aggregate of several factors including (but not limited to) the reflectance properties of the paint and the luminance variations observed from the illuminated topography of the stroke. Some people can spend a good deal of time trying to match a certain HVC (Hue, Value, Chroma) on the palette only to have the target color misread due to stroke orientation. Consider that varnishing a work can exacerbate the effect even further—increasing problematic reflections–making essential visual information very difficult to garner. The painter can contend with this in a number of ways, including changing the orientation of the stroke or using low-resistance (soft) brushes to carefully reduce the surface variations of the stroke itself.
In the graphic I prepared you can see how much variation can be observed due to stroke direction. (Granted, there is indeed some light fall off here, but it should not diminish the effectiveness of the graphic too much.) Strokes were averaged on the right so that you can get a better idea of the aggregate. As with most problems that painters face, awareness is the first step in diminishing the adverse impact of the issue.
Note: The stroke/mark orientation effects described here are also applicable to dry media works like charcoal and pastel.