The undertone of a specific paint color or hue is the color you see when the paint is spread very thinly. This can be done physically by brushing the color out thinly or by scraping with a painting knife, or by diluting the color with medium. The undertone is, obviously, most visible when painted on a white surface.
Some colors have similar mass tones and undertones, particularly colors that are very opaque (for instance cadmium red). Other hues have under tones that are quite different to their mass tone, something that's important to know when glazing as it influences the final colors you produce. Where you have colors that come in different shades, for instance phathlo blue green shade and red shade, the difference is most visible in the colors' undertones.
The color shown in the photo is Green Gold (an acrylic from Golden), which contains nickel azo yellow (PY150), phthalo green yellow shade(PG36), and Hansa yellow (PY3). The golden aspect to the color is far more evident in the undertone than in the thick paint (or mass tone).
