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Michele,
I also have to use Titanium White since lead is out of the marked here.
I always add some dryer to my medium ( I consider a medium being a mix of oil, turps and dryer )
I don't think that the small amounts of metal salts: lead, cobalt or manganese, that is required to accelerate the drying will affect the color, or cause darkening / yellowing of the paint.
The problem, as I have experienced it, is that we want the dark colors to stay transparent and therefore add more oil if the oil "sink" and leave the surface matte.
If the drying process is too slow then the oil will keep on sinking and we will keep on oiling out with the result of darkening the colors because of too much oil on the surface.
In other words it is the oil and not the pigment that causes darkening.
Over time the paint will get more transparent, so all supports should be primed in white. That would balance the expected darkening of the oils.
The white priming will do the job whatever or not we begin the painting with a wash of darker paint or not.
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