I got the impression (from both forum and general web searching) that there is no definitive answer to the question. So I thought I'd just test it myself.
I scribbled lines on a lightly-stained wood panel, using both the SoHo recommended yellow pencil, and a No. 2 pencil, as shown on 2017-07-12:
Then I painted over these lines using Geneva Titanium White, having cleaned the brush between applying the two swatches. This was also on 2017-07-12.
When I look at this on 2017-08-29, there is a distinct yellow tint to the No. 2 pencil (right hand side) swatch:
There are now dots on both samples, presumably dust in the air.
In general, it's not dots of pencil coloration, it's a general discoloration. This sample has sat in an open windowsill for six weeks.
Can anyone tell me, or guess at, what happened? Help me Obi Wan @dencal
, you're my only hope.