|Question: Should colour also be scaled?
Answer: Yes, there is a perfectly sound scientific basis for doing this.
The CIE (International Lighting Committee - the worldwide body which defines colorimetry standards) has two models of colour vision, the 2-degree and the 10-degree model. They are different, and relate to the perception of colour in the eye when the colour sample subtends either 2 or 10 degrees.
The bad news, on top of this, is that each of us perceives colour differently, even if you have "colour normal vision". The good news is that you get reliable and repeatable results from taking an average result from as few as 12 observers.
The other big effect on the perception of colour, which is relative to the colour of the surroundings in which the colour sample is viewed (hence the "change" which lining out can make to a colour), is the nature of the light under which something is viewed. For reliable colour reproduction all judgements should be made under standard conditions, and generally D65 is used, which is "Average northern-hemisphere daylight". This is not just a colour temperature (6504 Kelvin) but also a defined spectral distribution.
There is also the apparent difference in colour under different light sources which causes two colours which match under tungsten light to appear different under florescent, for example. You could easily have a rake of model coaches, painted with two different paints which appear the same shade on one layout, illuminated with one type of lighting, and different under another type of lighting.
All very hard for the lay-man, and after working in the field for 15 years, there is still plenty more for me to learn...
The main thing is that there is no way in which you should trust colour reproduction in photographs, especially old ones, ones in books or on web sites. I have even known people to judge shades of green from black-and-white photographs. If they knew anything about film emulsions etc they wouldn't contemplate attempting such a judgement! The only things one can really trust are the work of the likes of John Harvey of the SRG, and the HMRS LSWR/SR livery register, working from real measured paint samples taken from real coaches and locos, and even then you need to lighten the shade for model use to take account of scale.
Link to Station Colours website
Link to LSWR and SR colours Website
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This page was updated 31 May 2013