In my previous articles on reading and writing colours from various palette/swatch formats, I left CMYK conversion as an exercise for the reader and only demonstrated RGB aspects. This article demonstrates how to convert colours in CMYK format to RGB and vice versa.
(Image credit: SharkD [CC0], from Wikimedia Commons)
CMYK is traditionally used in the printing industry. It is a subtractive colour model and uses four colours (cyan, magenta, yellow and black). Unlike additive colour models (e.g. the RGB model normally discussed on this blog), combining all primary colours in subtractive results in black, whereas combining all primary colours in additive results in white.
Unlike RGB which generally uses the range
0-255, most examples
of CMYK I've seen use percentages instead, e.g. 37% cyan, 18%
magenta, no yellow and 31% black. In this article I'm using the
0-1 to describe the colours. (Just to be awkward, the
demonstration front end uses
0-100 to make it more user
As RGB uses completely different colour channels as well as different value ranges, conversions may not be directly compatible. For example, converting from RGB to CMYK and then back to RGB there is a chance to have a final RGB value that is just slightly off from the source. And that's before we even get into gamut and colour profiles, neither of which I'll be covering in this article.
Red is calculated from the cyan and black colours
R = 255 × (1-C) × (1-K)
Green is calculated from the magenta and black colours
G = 255 × (1-M) × (1-K)
Blue is calculated from the yellow and black colours
B = 255 × (1-Y) × (1-K)
Formula credit: RapidTables
The following function converts CMYK into RGB
The R,G,B values are divided by 255 to change the range from
R = R/255
G = G/255
B = B/255
The black key is calculated from the maximum red, green or blue colour
K = 1-max(R, G, B)
Cyan is calculated from red and black
C = (1-R-K) / (1-K)
Magenta is calculated from green and black
M = (1-G-K) / (1-K)
Yellow is calculated from blue and black
Y = (1-B-K) / (1-K)
Formula credit: RapidTables
The following functions convert RGB to CYMK
You might notice that it is possible for
(1 - k) to return 0
k is calculated to be solid black (
1). As this is using
floating point math, you don't get the normal
DivideByZeroException, but instead of the result of the call
ClampCmyk function make sure that values less
than zero are normalised as zero, and it also does the same for
I did write a version of the function that checked to see if
(1 - k) was
0 and then return zero for the
fields but the above version was easier to read.
The demonstration program lets you freely convert CMYK to RGB and RGB to CMYK uses the above functions, and a lot of sliders and spin buttons. I also included some predefined RGB colours, courtesy of Arne's 16 colour palette.
- 2018-07-15 - First published
- 2020-11-22 - Updated formatting
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