Whilst a cross section allows one to see the stratigraphy, each layer has to be exposed in order to determine the colour of individual schemes. However, this process will only make sense once a cross section has been made and the sequence is clearly understood. To carry out a scrape as the sole method of identifying a series of colours is folly.
The covering over of an early paint by later redecoration often had the effect of altering the original colour. Unless this is understood and allowed for, great distortions are likely, with blues being understood to be greens, and white as putty colour, for example.
It is virtually impossible to correlate the schemes applied to the different elements of a room, or series of rooms, when one carries out a paint scrape. Nor can any intelligent statement be made about the date of a particular layer.
An article expanding on this can be seen here.