The true colours on the subjects for our model creations vary. The colours are affected by weather and sun, usually they bleach over time. Wear and tear can alter the colours considerably. But if you're aiming for a not to used subject you usually want to get the colours as close to the original as possible. Not as easy as you might think.
The first hurdle is the build description. All of them give some reference to the colours to use. Older instructions might just state "brown", "green" or "light blue" for example in which case you really have to find some better references in books or on the internet (how did we manage without internet?). But today almost all instruction use some sort of colour reference number for at least one model paint manufacturer (some even give numbers to more than one manufacturer). This as such could be a challenge if your supply of model paints are of a different brand than the specified colours, but there are several paint conversion charts on the net (see my "Useful internet resources"-page) so through them or, in worst case through cross referencing them you should be able to find the right paint cannisters in your supply (or at your favourite model shop).
Or is it the right colour? Some model kit manufacturers are not careful when they pick the colours. When it comes to building old kits or really cheap ones it often pays to double check with some reference material. But most brands today put a bit more effort in getting it right. Sometimes this does not help though because you can not always trust the paint manufacturers either. If you really want to come close to the original it also pays to compare the paint to reference pictures.
I will demonstrate through a practical example here. Building the Messerschmitt Bf 110D I bought decals from Iliad Design from Hannants and the instructions to them had colours given in RLM numbers. Fine, just needed to translate them into the Vallejo Model Air equivalents and go. So I started with Vallejo's own "Equivalent Values"-chart and ended up with RLM76 = 71.046 and RLM74 = 71.054. OK, but the equivalent to RLM75 was a colour I didn't have so I hit the other reference charts and ended up with RLM75 = 71.052. But now I got suspicious because these colours didn't come close to the colour schemes in the instructions. Specially when it came to the top colours (052 and 054) that was virtually the same while even the black and white pictures of the real Bf 110 showed a clear shade difference.
What to do? By chance I found, in Vallejo's product catalogue a colour set called "Luftwaffe Colours 1941 To End-War". In this set there's totally other colours and RLM76 = 71.257 Light Blue, RLM74 = 71.258 Grey Green and RLM75 = 71.259 Grey Violet. When I got this set everything felt more correct and the end result using these colours finally looked OK. But still, if you trust Vallejo there is two different RLM76, RLM74 and RLM75 see them brush painted on paper below.
Getting it right is not as easy as it sounds.