We saw in Tone, Transparency and Saturation that although a fully saturated Lemon yellow and a Cadmium yellow with a touch of white may have the same tone, the former is transparent while the latter is opaque.  You will also find that once you have added white paint to another pigment it is extremely difficult to correct it and make that area of your portrait darker again.  Only a touch of white oil paint added to a dark colour will lighten it, but you have to add a lot of a dark colour to a whitened paint mixture to darken it again.  So, once you have put white on to your portrait painting, that area will be opaque and it will be pale and the only thing you can do to correct that is to wipe it all right off with thinner or, if you want to darken it, you may get away with holding a fully loaded brush virtually parallel to the canvas surface and applying a really thick coat of the dark with one stroke to cover it up (see Surfaces, Brushes and Brushstrokes).  The latter is fine if you are in the final stages of a portrait, but if you are not it may well create more problems than it solves because the paint will be very thick now and painting from ‘thin to thick’ or ‘lean to fat’ is always easier.  

A further property of white oil paint that separates it from some of the others is that it has a slow drying time.  Titanium white is particularly slow, Flake white paint is a bit quicker but both take of the order of days to weeks to become touch dry.  The earth colours (e.g. Umbers, Ochres and Siennas), and others (e.g. indigo), if applied reasonably thinly, will be touch dry for a second portrait sitting by the next day.  So you could, if you wanted to, apply a glaze the next day.  Depending on how you like to work, this may be another reason for working from darks to whites