Introduction

The colour appearance of any material depends upon the direction and composition of the light, the molecular structure and pigmentation of the material, and the distance from which its seen.  Further variables also arise from the area of the material in relation to the colours of adjoining materials, and the responses of the observer himself.  We may note here the phenomenon of colour constancy by which the brain registers the known local colour of the material seen regardless of the illumination of the surface at the time.  This can occur only when there is foreknowledge of the particular colour.  Strictly speaking the local colour of the material can only be known in certain controlled laboratory conditions: in practice it’s customary to make rather general assumptions.  In some cases a sample panel of the coloured material may be viewed on site throughout a whole year but this is rarely practicable; commonly it is for a few weeks or months.  Often it is not at all, and the colour is selected from a colour swatch or a piece of material.