Cause
Yellowing and discolouration of an oil based paint excluded from normal sunlight.
White and pastel colours are mostly affected.
A newly applied coating to oak timber styles showing tannic acid bleed .
Bleeding from a wood knot which was not sealed with shellac knotting
Bleed from the underlying coating being bitumen based and activated by the solvents in the finishing material.
Remedy
No solution for the those areas excluded from daylight other than using waterbased paints.
Where hardwood requires painting a leafing/flake base aluminium primer should be used.
Also see: Discolouration