Try using Phthalo Green Yellow Shade , which is much warmer, and therefore relates to the world and psyche better than Phthalo Green . Flake white is better because it does not crack. All white pigments have positive and negative characteristics. Flake White can crack in large cracks. Zinc becomes brittle.
Titanium looks chalky.
By combining titanium with zinc the paint tends to exhibit a balance between strength and flexibility and to be less prone to the negative characteristics of the pigments used solo. Since its ok to use acrylic gesso its ok to under paint oils with acrylic paints. As far as I know, there is no reason to believe that an oil painting will adhere long term to an acrylic underpainting. Good acrylic gesso is formulated to have absorbency and tooth, acrylic paint is not. The more expensive the brand of paint, the better it must be.