Hi, was the paintwork white before or was it stained, ie wood stain varnish etc. I know you say that you use Dulux but is it the One coat stuff like Once?
hi, i spent 17 yrs working in the laboratories of a paint company and if its going yellow quickly its because its cheap crap (to put it bluntly) and its not even always due to sunlight degrading it as some resins yellow more when in the dark.
you cant go wrong with dulux and they sell special non-yellowing versions, you get what you pay for!
I have gone onto egg shell magnolia, you don't get brush marks, just depends on what you want and if you prefer glossy or not , the art of painting is most definately in the preparation ......