White paint are very often used to tint Cadmium Yellow and Cadmium Red. The Ultramatine additive just increase Gray and you will have twice increased darkening effect. Use pure white like Permalba. If White paint have yellow tone, then use it to mix warm colors or muted colors. For superior violet and blue color, use Permalba White. Ultramarine additive is wrong for me.
I have read many times about such "magic" anti-yellowish ultramarine additive.
I found no logic there. If you use white to tint yellow, orange and red, then ultramarine additive is worst ever. If you use white to mix violet and blue, then ultramarine additive isn't necessary because you already have violet or blue in mixture. You will get a negative bonus!
I think, that best safflower Zinc-Titanium White paint has enough purity for most artists!