Finding complementary colours

WORKING WITH COLOURS:
Bringing it all together

Did you know there is more than one way to define a set of complementary colours?

Details:

• High school and up

• Time: 60-90 minutes

• Learning Outcome: Identify complementary colours via afterimage, subtractively mixing paints, optically mixing spinning disks and additively mixing light beams.

• Colour Concept: Recognize that there is more than one set of complementary colours.

Materials:

• Blue, yellow and orange-coloured paints

• Palette for mixing paints

• Paper

• Outline for spinning disks

• LED flashlights

• Coloured filters (blue and yellow)

Instructions:

Part 1: Finding complementary colours via afterimage

• On a white piece of paper, paint a square or circular area about 2-inches wide using your blue-coloured paint.

• Generate the afterimage for the blue coloured area by staring at the blue shape for 30 seconds, then looking to the blank white paper beside the blue shape.

• What do you see? Try to match the colour you see with other paints in your set. If you cannot exactly match the colour you see in the afterimage, describe how the afterimage colour and the painted colour are different.

• Repeat the exercise with a different blue-coloured paint.

Part 2: Finding complementary colours via subtractively mixing paints

• Choose a blue and a yellow or orange coloured paint from your set (suggestions: ultramarine and diarylide yellow, or cobalt blue and orange).

• You will mix your two chosen paints, and create a scale between them of equal visual steps (the number of steps is up to you – see Figure 2 for an example).

• Start with your yellow or orange, and paint a circle or square on the right side of your paper.

• On your palette, place small blobs of each paint, and mix a small amount of blue with yellow, until you have a uniform mixed colour. Paint a small area of this new colour on you paper, beside your pure yellow or orange shape.

• Repeat the process until you have 5 or 6 intermediate colours, between your yellow (or orange) and blue.

• If the two paints you chose are complementary colours, one of your mixed colours will yield an achromatic grey. In practice, this is hard to achieve. Identify your mixed square which is least chromatic. You may want to try the exercise again, with slightly different coloured paints, and see if you can mix an achromatic grey. 

Part 3: Finding complementary colours via optically mixing spinning disks

• Choose the same two paint colours you used in Part 2.

• Prepare a spinning disk, using the spinning disk template from Optical mixing using spinning disks, where the subtractively mixed colour is painted in the centre of the disk, and the individual colours are pained in alternating areas on the outer portion of the disk (see Figure 3).

• Spin the disk. What is the optically mixed colour? If you see an achromatic grey, they two colours painted on the outside are complements. If you do not see an achromatic grey, you can re-do the exercise with one or both of the following options:

  • Option 1: Change the proportions of coloured areas (you can create a new template for the spinning disk and use for example 2 yellow areas for each blue area, etc.).

  • Option 2: Use slightly different paints compared to your original choice.

• Which set of paints, and which proportions, gives you an optically mixed colour closest to an achromatic grey? If you are not able to mix an achromatic grey, describe how they colour you achieve is different.

Part 4: Finding complementary colours via additively mixing lights

• To determine complementary colours for the additive mixing process, two light beams will additively mix to an achromatic white. One way to demonstrate this is to use blue and yellow- coloured light beams.

• Using your white LED flashlights and blue and yellow-coloured filters, create a simple additive mixture of blue and yellow light as shown in Figure 4.

• Alternatively, you can create a yellow-coloured light beam by overlapping your red and green LED lights, and additively mix with the blue LED flashlight beam. What mixed colour do you see? (This exercise is best performed in a dark room.)

Vocabulary:

• Complementary colour, Afterimage, Subtractive mixing process, Additive mixing process, Optical mixing process

Questions & observations:

• Are all the mixed colours as you expected?

• Are all sets of complementary colours the same for each mixing process? If not, how do they differ?

• Were you able to create achromatic mixtures in each case? If not, how does your mixture need to be adjusted?

What’s going on?

• Complementary colours are pairs of colours that can be said to ‘complete’ each other. A complementary relationship can be established in different ways, two being widely accepted. By one definition, two paints, inks, or coloured lights are complementary if their mixture can yield an achromatic black, grey or white. By another definition, a colour and its afterimage are complementary. It is also common simply to claim that colours opposite to each other on a colour wheel/hue circle are complementary, without further explanation or justification, however as there are multiple ways to depict colour wheels, complementary sets are not unique. The difficulty here is that the different ways of defining complementary relationships yield different pairings.

• The most extreme case is with blue. To produce an achromatic or near-neutral dark grey by the subtractive mixing process, paint that appears blue must be mixed with paint that appears reddish-orange. To produce an achromatic white by the additive mixing process, a light that appears blue needs to be mixed with one that appears yellow. The afterimage of blue is a yellowish orange. So the complementary of blue can be red-orange, yellow, or yellow-orange, depending on how the complementary relationship is defined.

• It is important to recognize that complementary pairs are not unique, and can be defined in multiple ways.

Find out more:

• See glossary entry on Complementary colours