A Method of Simulating Paint Mixing on Computer Monitors Ferdinand Carabott & Garth Lewis St Martins School of Fasfion & Design The London Institute, U.K. Mixing coloured pigments is fundamental to an artist's practice. New colours are obtained from combinations of two or more colours e.g. mixing orange with blue produces green in the middle of a progressive mix. Computer programs like Adobe Photoshop can generate a mixture of two 'computer' colours by using the 'gradient' control in the toolbar. This blends one colour progressively to another. However, if one creates such a gradient in Photoshop using orange and blue as parent colours, no green is produced - instead a muddy brownish hue is produced in the middle of the gradient. Moreover, the Photoshop gradient mix produces a middle colour which is significantly lighter than what one would get with real pigments. Colour for computer painting programs is organised around a conventional colour space. Adobe Photoshop utilises the Lab colour space whereby any colour can be located by reference to three figures: L, a and b. As colours approach white or black, the circle of possible colours is ever diminishing, eventually converging to a single point of pure white or black when L=100 or 0 respectively. If we generate a gradient going from, for example, orange to blue, with the "Gradient' tool, Photoshop will create a straight line path joining these parent colours within the Lab colour space. It can be shown that this path passes through brown territory with no identifiable green. If, on the other hand, a swatch of colours made from a progressive paint mix from orange to blue is measured with a spectrophotometer and mapped on the Lab space, the path of the paint mixture follows a curve bending into green territory. Apart from orange and blue, ten other commonly used artists' colours were mixed in stepped swatches with each other. The intervening mixtures were all mapped into Lab space using a spectrophotometer. It became evident that most colour combinations follow curves with only very few following relatively straight lines. It's clear from these findings that gradients created in Photoshop will not be able to simulate paint mixes. We have devised a method whereby the straight Photoshop gradient line joining two parent colours is 'bent' to follow the 'real' paint mixes closely not just in hue but in value as well as mentioned earlier. It was also evident from paint tests that when mixed with other colours, white and black changed the hue as well as value - in effect black and white needed to be treated as 'paint colours' and had to be mixed with all the other 11 hues and their curves plotted into Lab space as well. All the colour combinations together with the black and white mixes are made available for use with Photoshop and thus comprise a colour library in two parts: 1. Colours imported into Photoshop as "Swatches". Such palettes can be saved permanently in the Photoshop folder resulting in over a thousand spot colours. 2. The entire palette of simulated paint gradients can be stored in the Photoshop folder. Any colour on the gradient can be spot-sampled and a mask can be switched on ond off to view the colours as steps, helping hue discrimination. Any colour-combination file or number of files can therefore be opened and viewed alongside artwork. In addition, a stand alone computer interface has been designed to allow simulation of colour mixing: all thirteen colours are shown at either end of the interface allowing choice of two parent colours to generate a stepped simulated mixture. This is useful short cut device to allow rapid experimental mixtures facilitating colour choice when working with computer graphics. The educational applications of this work range from the simple interface to a sophisticated colour course. Illustrators, graphic designers, computer artists and architects would also find this a useful tool bridging the seemingly disparate disciplines of analogue and digital paint mixing.