Phthalo is the painter's equivalent of nitroglycerine. You should probably need a permit to use it.
When I tell the students who've not used it before to add just the tiniest bit, they usually scoop up a three-month supply on their brush. The results are disastrous, of course. Unless you're skilled enough to use a single hair of your brush to pick up the phthalo, you're better off using a knife.
For those of you who have trouble managing phthalo, I include the following illustrations. The first photo shows a pinhead's worth of phthalo emerald on the knife. The second photo shows that pinhead's worth mixed into white. The third photo shows a second pinhead's worth mixed in. It's pretty dramatic.
By the way, I've set up a coupon that will give you $1 off my $10 online video course, Plein Air Essentials - Oil Supplement. This course includes not just several mini-videos but also a 30-minute oil demonstration. The coupon code is PAEOS and is good only until July 21. Go to this link for the course.