The full strength surface color of a paint as opposed to its appearance when spread very thinly as in gazing, or scraped on with a knife. For some colors there is no difference, but for some there is considerable difference. Cobalt Yellow (also called Aureolin) has a leathery brownish mass tone, but a bright golden yellow undertone, whereas Cadmium Yellow looks very similar applied thickly or thinly. Pthalo Green in mass tone is very dark whereas it’s undertone is a beautiful emerald green. This is especially noticeable in the yellow shade of Pthalo green.