In this post, I'm going to be comparing ultramarine blue to phthalo blue. I don't think either of these colors are better or worse than the other and I think there's room for both of them in any artist's paintbox, but there is a major difference between them that I want to point out.
I took a creative painting class awhile ago and it opened me up to some new colors, of which phthalo blue was one. The teacher told us it was very strong and he was right. The two pics below will demonstrate what I mean.
Here's some ultramarine blue mixed with titanium white,
and here's the same amount of phthalo blue mixed into some titanium white.
In conclusion, while I think phthalo blue is a lovely color, if you need a truly light blue, I would recommend ultramarine, cobalt, or cerulean over phthalo. When I use phthalo, it seems all it takes a little pin drop and, all of a sudden, you have this really rich blue, which can be lovely, but sometimes you don't want something that bright.
That's all for now. I'll talk to you again next Monday.
Painter of portraits and wildlife