CMYK and printing

Before Christmas we cast cyan, magenta and yellow shadows using red, green and blue lights. This was because of additive colour mixing - for example, if you add red and blue light together you make magenta. It made me realise that there was another activity we could do to explore how printing works with cyan, magenta, yellow and black (CMYK, where K is black or 'key') ink. It's taken me a while to get around to making something that I thought the small boy would find interesting, and it was only because he asked me last week about how our printer worked that I remembered my earlier thoughts and bought some acetate sheets for the job.

Inkjet printers have four colours of ink (some have three and make black from the others) - cyan (a bright, light blue colour), magenta (bright pink) and yellow plus black. They make all the colours you choose to print by mixing these inks. For example, to make red you actually print both magenta and yellow. This is known as subtractive colour mixing because unlike with our light mixing, the pigments in the ink absorb wavelengths of light rather than adding them.

Cyan, magenta and yellow rectangles on acetate

To show how the cyan, magenta and yellow inks work, I printed some rectangles of the colours as well as a grid of black outlines and colour names onto uncoloured transparent acetate and cut them into four strips. When all four are overlaid you see a box of each of cyan, magenta, yellow, red, green and blue. The small boy was tired after a session at preschool and I thought something different might improve his mood. I put the strips of acetate on a sheet of white paper and invited him to see what he could do with them. He wasn't keen to try so I showed him how to overlay them and he saw their potential. He then got stuck in lining them up and was happy to tell me about what he'd made! He had remembered the name of magenta from our last activity but not cyan, and when I prompted a little he could tell me that when he put two colours together he made new ones. We explored which colours had to be mixed to give each of red, green and blue and then I explained that each strip was printed with one of the inks in our printer and this is how it makes colours.

Overlaid cyan, magenta and yellow acetates with labelled boxes

He wanted to know if you could make more colours with them, and for once I was prepared for his question! I'd separated a photo - of the colourful elephant cake which he and I made for his brother's birthday - into the CMYK components and printed each colour ink on a separate piece of acetate. I'm sure this process is trivial if you have Photoshop or similar, but I couldn't find an easy way to do it with the limited image editing software I've got so I made use of a couple of free online tools. None did exactly what I wanted but with a combination I got there. As it was fiddly, I've put a PDF of all the acetate prints mentioned in this blog here for download in case you want to try this activity without the faff of making your own.  The small boy was initially puzzled by the 4 different coloured versions of the cake picture but he tried to overlay them and - with a little help to line them up fully - he discovered the Elmer elephant cake! He was really excited to see all the colours appear as he put the last acetate sheet on the top, and he played with it for a few minutes, exploring what happened to the colours when he moved individual layers. He was also keen to show the acetate colours to his Dad when he came home from work, which is usually the sign of an activity that he's really engaged with.

He made an image of the cake by overlaying the four CMYK prints

The other thing I printed on the single sheet of A4 acetate was some large ovals of each of the CMY and RGB (red, green and blue) colours so that he could do some more exploration of the colours he could create with overlapping them.  These didn't amuse him for as long as Elmer, but he did ask to come back to it the next day.  He found that it's possible to overlay cyan, magenta and yellow such that you can see red, green, blue where pairs of colours overlap and black when all three coincide.

Overlaid colours

As a bit of an experiment in how I can use this blog, I've put a copy of the single A4 layout I used to print the coloured pieces of acetate I've referred to above.  If you try downloading it, please let me know via comments if it works for you, and if you find it useful. If the link works, I can add downloadable files to future blog posts where they make it easier for others to try our activities.

We also went on a hunt for evidence that commercial printers work the same way and found, under various flaps on packets of cereal and other food items, colour test strips showing the four CMYK colours of ink.  My son was pretty unimpressed by this, but he did point one out to me when he saw it in a shop later, so he'd clearly taken it in! Hopefully this activity answered his question about the printer, now I just need to think of some hands on ways to answer some of his other 'how does it work?' and 'why?' questions!