In this generally confused and poorly written article, (
https://www.duluxtradepaintexpert.co.uk/editorial/dulux-trade-high-gloss-pure-brilliant-white including the statements: "In this review I've been looking at the Dulux trade high-gloss. I've been using this product for around 15 years and due to different European regulations - mainly VOC 2010, paint has changed a fair bit in this time.") - by "Product" the writer means "Brand" - the paint product today bears little relationship to the Dulux paint products of 15, 30, and 60 years ago, and all the intermediate evolutionary versions - in the final paragraph it states:
"Note: Whilst leading-edge technology has been used in the development of this product, the nature of solvent-based paints means that they will all discolour over time.
The rate of discolouration is faster in areas with no natural or UV light. For long-lasting whiteness, we recommend the use of a water-based formulation such as Dulux Trade Ecosure Water-Based Gloss."
The bolded text does not state, as it might have done, that the discolouration is bleachable.
Now, if Dulux had been smart, and out for an extra buck or two, they should have released "specialist" paint care kits that were recommended to maintain and clean (i.e.
bleach ) the paint work every 3 months or so, with a discount voucher for the starter kit with the original paint purchase, and cleaning products etc., with refills for continued brand loyalty.
The kits, for instance, could have a safe UV light source, and a UV activated gel (which would release a concentrated chemical bleaching agent) and applicator. They would work on the principle that molecular oxygen absorbs UV energy and dissociates to give energetic oxygen free radicals which may oxidise and thus bleach chemical bonds that produce colour (i.e., those bonds that cause a once transparent medium to absorb light and darken in certain wavelengths) in organic polymeric compounds (epoxides) fundamental to the "glossiness" in gloss paints.
As it is, a cheap supermarket thick bleach, applied neat (with due care regarding spillage, ventilation, and protective clothing to prevent exposure to skin, eyes and airways) to indoor paintwork with a nylon bristle paintbrush, at about 90 day intervals will largely remove yellowing, and many other stains like smoke and cooking volatiles, mould growth etc. It can be allowed to dry for maximum effect, and any residue wiped off with a damp cloth, polished with a dry cloth with no loss of paintwork glossiness, and no loss of original colour in coloured paintwork - which has had bleach (i.e. UV, or chemically induced) colour resistance for many years. Discoloured paint that has never been bleached will show dramatic, although rarely total, bleaching of the discolouration.
If you test patches of paintwork in shady and secluded room corners which are not exposed to bright lighting you can estimate the potential benefits of using a bleach on the paintwork, instead of, or before redecorating, maybe saving a lot of money on paint and materials.