I posted this on an art forum a little earlier in response to a thread asking if the long held belief that zinc white causing mixes to be “cooler” (i.e. bluer) than when using titanium white is correct.
Titanium white (PW6) is much stronger so it only takes a small amount to have a big effect on the paint you’re tinting with it. It’s more opaque and is often times said by artists (from what I’ve seen especially by those who prefer lead white, such as PW1) to make tints seem “chalky” or dull. In this comparison I’m also trying to see if this is the case.
Zinc white (PW4) is more transparent and much weaker when it comes to lightening other paints in tints, so you’ll need more of it to have the same effect. For this reason it’s sometimes called mixing white, because you can more easily make small adjustments to a paint’s lightness without accidentally making it too light. It’s also said to become hard and brittle if allowed to dry on a canvas by itself and not mixed into anything else.
This was photographed yesterday in late afternoon outdoor sunlight. The image should be mostly self explanatory. I mixed each row to be about the same value (amount of lightness/darkness), and it did take far more of the zinc white to get the same result.
The color samples were averaged in Photoshop over a 31x31px area of each swatch and the results are shown in Lab color space.
As you can see, the “cooling” effect of zinc white was minimal, being only noticeable by a small amount in the middle and dark values of grey. I chose charcoal black because it’s close to being neutral black, so I thought if anything would show a slight blue shift then that would be it.
The blue showed almost no difference between the two in the photo, although in real life I can see a slight difference with the lighter value being a little duller on the titanium side.
The red was easily the biggest difference, with the lighter value being very noticeably duller on the titanium side. Interestingly, the green/red results had almost no difference but blue/yellow did.
It appears people who say titanium white makes chalky tints actually may have a point, as it definitely affected the red.
This photo was taken the next day in about the same lighting as the previous one. I thought there’d be a difference in the greens or yellow, but they’re almost 100% exactly the same. I got most rows to be nearly the same value, except the last couple on blue. This time some samples were averaged from 11×11 squares because small ridges of paint across a few samples that might throw off the accuracy.
The conclusion that I’m coming to includes three things-
1. The idea that zinc white “cools” colors in mixes doesn’t seem entirely true, at least not with this tube.
2. The idea that titanium white produces “chalky” tints also seems to be false, given that most of the tints from both tests are nearly the exact same whether it was made with titanium or zinc. Maybe the people who say it’s chalky are comparing it to a lead white, but I avoid lead paint and can’t do a proper comparison with it.
3. Either of the above conclusions may have specific exceptions, and those alone may be the origin of these ideas. The lightest cadmium vermilion tint was much duller with the titanium than zinc, and the charcoal black did show a small shift toward blue with zinc.
My camera itself and the lighting may also have played a role. Even so, from what I’m seeing here it seems that there is in most cases no more than a minuscule difference between zinc and titanium whites when both mixes have been mixed to the same value, and many cases where there is no visible difference.
Too Much White Paper 
Really good research, will use as a reference. Thanks for taking the time to do and share. One thing I’ll add: In my experience, the “chalky” effect of Titanium becomes most apparent in layer transparencies/translucencies and glazes. In most glazes, I find a small amount of white is needed to avoid a progressive darkening with the layering. This is where the difference between the two pigments is most noticeable: the titanium gets milky, the zinc oxide not so much. I think some of this may be attributable to the odd feature of zinc oxide (strangely not discussed much), in which it has different reflectivity at different angles, appearing almost iridescent in washes from oblique angles.