When trying to buy paint, the biggest imponderable is how the new paint will cover the old surface. This can be 300% out if you are about to splash white emulsion on a fresh plaster coating.
Today’s invention is a handheld tool consisting of hygrometer sensor, of the type used to detect damp in walls, plugged into a cellphone (with camera).

A program on the phone directs the user to take a picture of the colour
of the existing wall (under normal light conditions). The moisture sensor is jabbed into the wall surface (somewhere unobtrusive).
The phone then requests an image of the paintcan barcode (which can be done in the shop, before any purchase is made, to allow comparison of the severals shades of ‘dusky pink’ on offer).
The program can then advise on how much of the paint will be required to cover a square metre of wall surface.