Thanks to Neil for this one! This bluish red is one of the most useful modern paints to exist primarily due to it's luscious transparency and incredible vibrancy. Essential for portraiture, please read the following for Natural Pigments fascinating description of this paint, it's lightfastness and use as a glaze.
"Franz Hals was well aware of the correct way in which to use fugitive lakes in order to produce vivid flesh tints. The ruddy noses and rosy cheeks portrayed by him which after a lapse of centuries have shown that this colour, when intelligently used, is stable. According to Maximilian Toch, if madder lake (the natural lake pigment related to alizarin) is used as a glaze over a color which has been allowed to dry thoroughly, it will remain permanent, but if mixed with any mineral pigment, such as ocher, lead, sienna, etc., it is altered and loses color. This Toch based on his observation of Rembrandt's use of the color in works like the Anatomy Lesson, in which 'upon close examination the flesh tones have suffered very likely due to the fact that the lake used in glazing was mixed with the under-coat.' Based on this observation it is, therefore, reasonable to assume that alizarin or madder lakes should be used as glazing colors over a properly dried surface. Alizarin or madder lakes will not decompose when mixed with various blacks such as bone black, lamp black and carbon black.
Vermeer more than once placed a glaze of lake pigment over vermilion. He first modeled the object to be represented in various tones of vermilion using white to lighten the tone and black to darken them. Once the area was dry he glazed over it with a lake."