What is RGB colour? What is CMYK? and what is the difference between RGB and CMYK? It’s essential to know this, if you need to work with images on the web and in print.

1. COLOUR PART 1: THE DIFFERENCE
BETWEEN RGB AND CMYK

2. In my previous post on how to save images for the web
with Photoshop I emphasised that you must make sure
the image colour mode is RGB, and not CMYK, when
you save for the web.
In this post you can find out what the difference is
between the RGB and CMYK colour systems, and why
it is important to know it.

3. What is RGB colour? What is CMYK? and what
is the difference between RGB and CMYK? It’s
essential to know this, if you need to work with
images on the web and in print.
It is essential to understand that colour is created by light.
Without light, we cannot see colour.
Simply put: colour is perceived by the eye as a result of
the way an object emits or reflects light.
Colour in itself doesn’t exist: it’s a human perception
COLOUR IS LIGHT

4. of wavelengths of electromagnetic energy that make
up the visible spectrum of light. This perception also
varies depending on a number of factors: physiology,
psychology, culture.
This applies both to the colours that you see on a screen,
and the colours of any reflective surface – in the first case
the colour is emitted, in the second the colour is light
reflected on the surface.
Let’s have a look at how that works.
COLOUR IS LIGHT

5. THE RGB COLOUR SYSTEM
In the image below, you can see a visual representation of
the RGB colour system, in which colour is created by light.

6. The RGB acronym stands for Red, Green and Blue: the
primary colours for the RGB system.
When they overlap, they form white.
This is why they are called additive colours: they are colours
obtained by emitted light directly from a source, and when the
three primaries overlap, the result is white light.
RGB colour is made of LIGHT. Red, Green and Blue are
LIGHT BEAMS.
THE RGB COLOUR SYSTEM

7. So, right off, this is the difference between RGB and
CMYK.
In the RGB colour system the colour is emitted directly, so
it shines through.
In the CMYK colour system, the light is bounced off the
surface. It’s not transparent: it’s reflective.
THE RGB COLOUR SYSTEM

8. RGB is the colour system used on your computer screen
and on your tv, on your telephone and tablet, at the
cinema, on your camera.
Any time it involves a digital platform – it’s RGB.
Simple.
There are 256 colour levels (0 to 255) in the RGB colour
system. Thanks to this, we can form other colours.
THE RGB COLOUR SYSTEM

9. This is what the RGB colour slider looks like in Photoshop:
In this example, 0 red, 128
green and 120 blue give a
teal green colour.
THE RGB COLOUR SYSTEM

10. Below, you can see what happens when you have a full beam
(255) for each of the primary colours: white is the result.
It’s the linear representation of the
three light beams overlapping that
we saw at the beginning.
The addition of the three primaries
creates white – this is why the RGB
colour system is called additive.
THE RGB COLOUR SYSTEM

11. On the other hand, zero red, zero blue and zero green
produce black.
Black is nothing other than
the absence of light.
Zero light = black.
THE RGB COLOUR SYSTEM

12. So this is why, when you save for the web, you must make sure
that your image is saved as RGB. This is the colour system that
your screen, and your browser, and the internet, will understand.
The RGB colour system comprises a very wide range of colours
(called gamut): if you multiply the levels of the 3 RGB values, 0
to 255, by one another, you get nearly 17 million colours:
256x256x256 equals 16,777,216.
THE RGB COLOUR SYSTEM

13. Things are quite different in the real world, where most
objects get their colour through a combination of reflection,
absorption and scattering of the light that hits on them.
This is why the behaviour of colours changes entirely
when you print things out.
The first reason is that, as we saw earlier, colours in the
real world, outside of screens, are mostly produced by light
bouncing off surfaces, not shining through them (unless it’s
glass or other see-through surface of course).
THE CMYK COLOUR SYSTEM

14. THE CMYK COLOUR SYSTEM

15. The second reason is that in order to print colours you need
to use coloured ink.
The colours of these inks are: cyan (blue), magenta (a bright
pink), yellow and key (black).
This is what the CMYK acronym stands for. All colours in
print are formed by mixing a percentage of these 4 colours.
The third reason is that the colour of the paper you use will
also produce a variation in colour.
THE CMYK COLOUR SYSTEM

16. The CMYK colour system is called subtractive, because each
of the colours when printed subtracts from white.
Also, when these 4 colours overlap they form 100% black –
so, the opposite of RGB, where the 3 colours overlapping
forms white.
THE CMYK COLOUR SYSTEM

17. THE CMYK COLOUR SYSTEM

18. The same teal colour we saw above, translated from RGB
to CMYK:
You can see from this colour
slider how you can mix colours in
CMYK:
86% blue, 30% magenta, 55%
yellow and 9% black give you the
same teal green colour.
THE CMYK COLOUR SYSTEM

19. It’s worth repeating, however, that the colour you see on
your screen will not be the same when printed.
Colour transferred from on-screen RGB to print CMYK
changes considerably: don’t be disappointed when it does.
Always ask your printer, or print a sample before.
THE CMYK COLOUR SYSTEM

20. It’s also best to use round numbers when you make up
CMYK colours: always round up the percentage to the nearest
5% or 10%.
So for that teal colour I would probably advise:
C85, M30, y55, K10.
Graphic designers often use big printed swatch books so they
don’t get their colours wrong once they are printed – it would
be a costly mistake. Much better to make sure they know what
you are getting, before you go to print.
THE CMYK COLOUR SYSTEM

21. This is an enlarged example of how the CMYK printing
process works: a photographic image is separated into the 4
primaries CMYK, in the form of small dots.
THE CMYK COLOUR SYSTEM

22. This colour system, as said before, is called subtractive,
because each of the colours when printed subtracts from
white.
Also, when these 4 colours overlap they form 100% black
– so, the opposite of RGB, where the 3 colours overlapping
forms white.
THE CMYK COLOUR SYSTEM

23. THANK YOU. I HOPE YOU
FOUND THIS USEFUL.
YOU CAN READ THIS POST ON MY WEBSITE, TOO.
IF YOU ARE INTERESTED IN COLOUR, YOU CAN ALSO
DOWNLOAD THESE
2 X COLOUR CHEAT SHEETS.
THANK YOU!