I used to squeeze out ultramarine in my paintbox palette. It’s a great color! But it doesn’t LOOK blue the way cobalt does.
Even though I’m rigorous about putting my paints in a rainbow pattern, always using the same placement, there is nevertheless confusion in my paint box. Many pigments are so dark that I rely on their placement to figure out what I’m getting: hooker’s green, thalo blue, dioxazine purple.
However cobalt blue is one color, like permanent green, that looks like what you get.
When I reach for it, I don’t have to think or hesitate.
One day I realized I never reached for ultramarine any more.
Cobalt is expensive but it’s a lovely true blue and makes the process of painting more streamlined.
Also, going for thalo blue is time consuming. One has to be careful to get just a smidgen of this intensely staining color on the brush, then wipe it off somewhere on the palette before introducing just a breath of it to the paint mixture. By the time I get a bit of thalo blue, add white, and mix in the ultramarine — I have spent a minute or two mixing something I can get by a simple dip into cobalt blue.