What would be your best advice to stop chalked paint yellowing?

Most white paint will yellow slightly with time, unfortunately. Water-based paints tend to be reactive and more likely to draw out tannins or other substances in the wood which causes the yellowing over time. I would apply one or two coats of shellac before painting to block any tannins that can bleed through

Thank you, for your help, it seems to happen quicken then I would think but thanks I will try that

cheers

Juanita