Gamblin Artist Grade Oils - Ultramarine Blue
Ultramarine Blue is an essential for every oil paintbox. Because there is no perfectly primary mixing blue, this is one of the two blues you will need for a primary palette. Ultramarine Blue is almost primary, but has a trace of red in it. You will also need to buy Phthalo Blue, which is almost primary, but with a trace of yellow. Since almost all things in nature have a trace of red, your ultramarine is your go-to blue primary, fitting the bill for almost all destination colours. For colours without any red, like high-key turquoises, Phthalo Blue is your ticket. If you are trying to get a primary blue, mix Ultramarine and Phthalo Blues together, and the traces of red and yellow will be virtually undetectable.
Both colours are transparent, and mix well with white to create opaque blues. They, of course are excellent glazing colours.