How to make Navajo White paint less yellow? see below?

Theoretically, adding purple or blue would make for less yellow. That, however, is really only true if you're dealing with pure colours of which you know the exact shades. You're dealing with paint, and you don't really know exactly what colours are in it. You're just as likely to create a grey tone as to eliminate some of the yellow. Try it with a small amount and see what happens.

Throw it away.  Prime and paint with a new color that you LIKE.

The only way to lessen the yellow is to add the opposite color in equal amounts. The opposite color on the color wheel is Purple.  When you add purple to Yellow you get a medium brown or tan. So, if you add just a small (!) amount of purple, your white will appear more tan than yellow.