In my experience, Phthalo blue (pronounced thay -low ) is one of the hardest colors to control.
A well-appointed palette for the landscape painter will allow you to mix just about any color you need. My expanded primaries palette is one such palette. An essential color on that palette is phthalo blue (pronounced: tha-low) , that rich and delicious shade of blue that is able to produce the brilliant “sky blue” color we are all so familiar with.
The problem with phthalo is that it is so intense and strong, that even the smallest amount can overpower any mixture. If you were to replace ultramarine with phthalo for most of your blue mixes, your painting would take on a strange, acidic blue-green pall. Another alternative to phthalo is manganese blue hue. Straight from the tube, manganese is about the same mid-value as Mediterranean and Azure blues. However, it is not as potent as Mediterranean, Azure, or phthalo. This means that you can use it without concern that it will overpower your mixtures.