Electronic colour measurement and conversions
Resene RGB values are measured from dry painted colour samples using sophisticated electronic colour scanning equipment. These RGB values are then entered into Resene Find-A-Colour, an electronic colour swatch programme to give a converted LAB value and a converted CMYK value.
The number of discrete colours available in the RGB space is over 16 million. In the LAB colourspace, the L can have integer values from 0 to 100 (101 discrete values), the A and B both can have integer values from -120 to +120 (241 discrete values each). Multiplying these together, the LAB mode has only 5 million discrete colours available. The LAB gamut is larger than the RGB colourspace, so when RGB is converted into LAB. the L will be utilised fully (0 to 100) the A will only have values from 99 to -90 and B will only have values from 92 to -107, which means the 16 million RGB values will be mapped into just under 4 million discrete colours in the LAB space.
The LAB mode is very non-linear so the mapping from RGB to LAB is not necessarily consistent across the colour spectrum so some colours will remain intact, some will change a little and some will have noticeable differences.
The same kind of effect happens when moving from RGB to CMYK. You can convert from RGB to CMYK but the process is not reversible in that you can go from CMYK to RGB but you are unlikely to get the same initial RGB values due to the colour shifts during the conversion process. These differences are not faults in the conversion or software, but purely limitations of mapping dissimilar colourspaces to each other.
Because of these shifts in conversion, colours can be converted from RGB to CMYK and LAB, but when converted back they may result in different values due to the mapping process.
Mixing paint colours is called subtractive colour. This means that the colour you see is the colour that the paint does not absorb. For example yellow paint appears yellow because it absorbs all colours except yellow. Mixing light colours (e.g. RGB values) is called additive colour. This means that the object gives out (emits) colour, so that even if it is dark you can still see the colour. A television uses additive colour because it emits colour. Even if the room is completely dark if you turn the television on you can still see the television program. Due to these different colour systems and the limitations of the RGB colour system, the RGB colour system portrays some paint colours better than others.
Electronic colour is a superb tool for visualising the paint scheme prior to painting but must not be used as a substitute for the physical colours. In all cases Resene recommends that you view a physical sample of the colour, ideally by trialling with a Resene testpot where available, before making your final colour selection.