Enigmatic: difficult to interpret or understand; mysterious.
Chiaroscuro: an effect of contrasted light and shadow created by light falling unevenly or from a particular direction on something.
Analogous Color: Analogous colors are groups of three colors that are next to each other on the color wheel, with one being the dominant color, which tends to be a primary or secondary color, and one on either side of the color.
Earth Tones: is a color scheme that draws from a color palette of browns, tans, warm grays, and greens. The colors in an earth tone scheme are muted and flat in an emulation of the natural colors found in dirt, moss, trees and rocks.
Also, and I don’t know the exact terms but, like most portraits Mona is placed in the center of the portrait to grab your attention. There is also contrast between foreground and background, her, and then the rocks and hills fading into the background. Also, her face and hands form two more focal points, they are positioned - just - far enough apart that you more or less have to center on only one point at a time, face/smile, background, or hands.
Also, and I forget the name of the term, but the paint/oil was applied in many shallow layers, adding to the mysterious effect. Plus, the scenery, you can’t quite place “where” she is, that looks like “normal” hills and trees but it has a surreal effect to the geography as well.
And then of course, is her expression. Muted and subtle but conveying or implying…
But really, it is the mystery, the enigmatic effect. Movies like Mullhoand Drive or Under The Skin, movies where the exact meaning is unclear, are often times much more captivating, arresting, then simple and easy to understand movies.