White paint discolouration

3 months ago I painted a south facing window-cill with Dulux Brilliant white non-drip gloss. Now I notice that where there are items on the window-cill the paint has discoloured underneath them...Has anyone got any ideas as to why??
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If it was plant pots on the window sill, then the acids and salts from the soil will make loverly marks in the paint. If it was just ornaments and things, then it more than likely been polish or cleaning fluids that have stayed under them and not been wiped away. Or it could be that the surrounding area is bleached by the sun more than the areas that were covered by the items.
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Solvent paints often change shade over time, particularly in response to UV radiation from the sun. Consider using water based paints which generally seem to suffer less from this.
Christian.
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This is probably an old wive's tale, and I have certainly never tried it, but I was told if you add a small amount of BLACK gloss paint (e.g half a teaspoon full) to the white gloss and then mix it very well, it will never discolour. Haven't got the heart to try it myself, but maybe someone here and confirm or deny.
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