This is a yellow of extraordinary beauty, although it may not seem so at first. This might explain why it it was discovered in 1830 but sat on the laboratory shelf for more than 20 years before someone decided to try it out as an artists color. Watercolorists immediately fell in love with it because as soon as they added water to it they discovered a very special secret about the color. It has a leathery yellow mass-tone that looks a little dull in the tube but thin it down and it changes. This is because Aureolin has an unusual quality - when applied thinly or watered down to a watercolor-like wash the full beauty of the pigment is revealed in its under-tone. The watercolorist is only interested in that bright clean under-tone but the acrylic painter has the choice of utilizing the mass-tone/under-tone (the full strength surface colour of a pigment viewed by reflective light is called its mass or top tone, its colour effect when it is spread out thinly is called undertone) differences for some very beautiful effects. It does make the color a little difficult to definitively describe as it is almost like two colors in one but some general points can be noted. Firstly chemically it is a cobalt compound which means that while it is not as lightfast as Cobalt Blue, it is still amongst the more lightfast of the colors available to the artist. Secondly its beautiful, clean and transparent undertone makes it one of the few mineral colors that is well suited to glazing techniques. Unfortunately it is amongst the more expensive pigments but then no other pigment is quite like it and sometimes a color is so good it is worth the few extra dollars to get the real thing.