Just spoke with the local store manager with the Jerry's Artarama here in Houston. He is an artist as well that loves working with walnut oil products.
His comments regarding blending zinc and titanium whites was be very careful with the zinc, a 50/50 mix or a little more zinc should be ok, anything more will likely lead to cracking. Glazes with zinc should be out of the question for the same reason. He is now using Michael Harding custom paints, and has found that lead white allows for certain color blends (shades) that you can not get with the other whites.
Interesting conversation, if your not familiar with Jerry's, their pricing is very competitive and I personally prefer them to Blick's here in the US. Not sure about international shipping.
PS Adding Michael Harding's discussion of his various paints, consistent with Annie's initial discussion on various whites. Whites are discussed first, but left the remaining colors in in case you are interested.
I found this to be a very interesting discussion of the history of white paint.