Interactive Primary Newsletter 34 Nothin' but blue sky....

Mixing colours                             Paints and ink Ask anyone to name the primary colours and they generally answer red, yellow and blue. Some may also suggest that you can produce different colours, even white, by mixing red, blue and yellow paint or ink. Like most of life, in practice it is not quite so straightforward. We have all seen rainbows and understand to some extent that it is the sunlight (‘white light’ – Newton’s description) being split up into red, orange, yellow, green, blue, indigo and violet (ROYGBIV). However, all of us having dabbled with painting at some time in our lives know that there is no way you are going to get white if you mix all the colours in the paint palette together. The water pot you use to clean your brushes usually ends up a yucky brown colour. To understand why this happens you need to know the difference between the light emitted by a source and the coloration of a surface. The coloration of a surface (Fig. 11)  is reflected light and is due to the colour of the light shining on the surface and the optical properties of the surface. When light shines on a surface some of the light is absorbed and the rest is reflected. The colour seen is the colour of the reflected light. Figure 11 - Colours reflected from a car's surfaces   Try this simple, fun colour mixing process using coloured acetate strips (gels) placed on an overhead projector. All you need is an A4 piece of white paper, strips of coloured acetate and an overhead projector. Lay different coloured strips on top of each other and observe the ‘new’ colours produced. We took this a stage further by laying strips in rows to produce a patchwork effect (Fig. 12). In this instance the colours seen, result from the removal of colours from white using the coloured gels. Figure 12 - Acetate film patchwork Paints and ink What happens when you mix red, green and blue paint or ink – do you get white? No, you get black. This is because the red part of the mixture absorbs the non-red colours in the spectrum. The green absorbs the non-green colours and the blue takes out the non-blue colours. Therefore no light is reflected because all of the colours have been absorbed and the surface coloration is black. In practice, you may get the familiar yucky brown because not quite all of the light is absorbed (Fig. 13). Figure 13 - Mixing inks   Light sources