Subtractive mixing process
The subtractive mixing process can be thought of as a physical or external mixing process (compared to the additive mixing process, which can be thought of as a perceptual or internal mixing process). In this process, different colorants are combined, resulting in a mixture with a new colour.
Subtractively mixed colorants (or filters) remove light, each with a different range of wavelengths from the same light beam. The remaining light is reflected to our eyes, and our visual system interprets the light as the mixed colour.
For example, if we physically mix blue and yellow-coloured paints, each paint removes light with its own set of wavelengths from a light beam. The light reflected by the mixture enters our eye, and our visual system interprets the mixed paint colour as green.
Examples of various greens mixed from different blue and yellow-coloured paints. Image courtesy of Robin Kingsburgh.
The exact mixed colour will vary, depending on the type of paint used (e.g. oil vs acrylic), the specific paints used (e.g. phthalo blue vs ultramarine blue) and also the brand of paints used. Each of these impacts the way a particular paint absorbs and reflects light.
The mixed colour also depends on the relative proportions of paint.
Mixtures from varying proportions of Ultramarine and Diarylide Yellow. Image courtesy of Maggie Maggio.
Another example of the subtractive mixing process occurs when a single beam of white light is sent through stacked coloured filters. In this instance, instead of a coloured paint reflecting light, a coloured filter will transmit light with a particular range of wavelengths. The Figure below demonstrates what happens when a single beam of light is sent through blue and yellow-coloured filters. In this instance, light with middle wavelengths can pass through both filters most easily, resulting in the perception of green.
A single beam of white light passing through both blue and yellow-coloured filters produces a subtractively mixed green. Image courtesy of Maggie Maggio.
See also: Mixing processes.
