Approved Document M talks about the difference in Light Reflective Values (LRV): what is LRV and why is it important?

For people with adequate vision, differences in hue (the nature of the colour) or chroma (the intensity of the colour) will provide adequate visual contrast between colours. Unfortunately, this is not so for all people who are visually impaired. The main feature of a surface, which appears to be significanty associated with the ability of visually impaired people to identify differences in colour, is the amount of light the surface reflects, or its light reflectance value (LRV).
Light reflectance value (LRV) is the total visible light reflected by a surface at all wavelengths and directions when illuminated by a light source.

The differences in LRV can be used to ensure that surfaces and fittings and fixtures provide sufficient contrast with the surrounding areas to make them more easily identified by those with impaired eyesight.

The LRV scale runs from 0 to 100 points. To rate 0 a surface would reflect no light at all and a surface that has a value of 100 would absorb no light at all; i.e. pure black and white respectively. In practice these extremes pure colours are never reached.

Research suggest that a LRV point difference of 20 may provide enough contrast for visually impaired people to discern a difference between colours. However a 30 point difference is generally recommended: indeed this is the value stated in AD M.