Colours

Colour Theory and Application

B.Sc. (Hons) Multimedia Computing Media Technologies

Agenda

 Describing Colour
 Colour Models and Synthesis
 Colour Palettes
 Colours for the Web


Describing Colour

 Words we might use when naming and
describing colours:
 ‘shade, hue, warm, tone, bright, dim, pale,
intense, vivid, rich, saturated, cold, strong,
wishy-washy, glowing, colour, flat, weak,
muddy, soft, vibrant, luminance, bold, tint’
 Very subjective - influenced by perception and
characteristics of illuminating light source.


Using the Colour Wheel
 Colour Wheel video
 http://www.youtube.com/watch?v=59QGexKdFUI

What about text?
 A useful interactive tool:
 http://www.hgrebdes.com/colour/spectrum
 So, text is different!
 You need contrast – but not too much.

Colour Models:RGB

 Display devices generally use a color model
called RGB.
 It stands for Red-Green-Blue
 Based on the additive synthesis model
 RGB colour value specified in three bytes


Colour Models: RGB

 Each RGB value is given a number from 0 –
255 starting from Black: R = 0 , B = 0, G = 0 to
White: R = 255 B = 255 B = 255
 Thus 256 colours can be represented by one
byte = 2 8
 Total possible colours from RGB code is 255 x
255 x 255 = 16 581 375 colours = True colour,
but will require 3 bytes per pixel ( 24 bit
colour ).


How’s your Hexadecimal?
 Base 10 digits= 0 -> 9
 Base 2 digits = 0 -> 1
 Base 16 digits = 0 -> F
 Why?
 So each byte goes from 00 -> FF
 So FF FF FF (R,G,B) = ?
 And 00 00 00 =?
 80 80 80 = ?
 FF 80 80 = ?

Colour Models: Hue,
Saturation, Value (HSV)
The HSV model is based on the following concepts
 Hue - this is what most people would refer to as the
colour or shade. Red, yellow, green and blue are
hues.
 Saturation - this is a value that represents where the
colour is on a scale from achromatic white to the pure
hue.
 Value - is the attribute, sometimes referred to as
brightness, which determines how intense the colour
is on a a scale from black to the pure hue.

The Colour Picker (Maya)


Colour Models: Synthesis

 Additive Synthesis - Start with no light. Add red
green and blue light to make white and
complementary colours
 Subtractive Synthesis start with white light.
Subtract red green and blue light to achieve
complementary colours and black


Additive Synthesis


RGB Additive
 Additive color: Combine light sources,
starting with darkness (black).
 The additive primary colors are red (R), green
(G), and blue (B).
 Adding R and G light makes yellow (Y).
Similarly, G + B = cyan (C) and R + B =
magenta (M).
 Combining all three additive primaries makes
white.


Subtractive Synthesis


CMYK Subtractive
 Subtractive color: Illuminate objects that
contain dyes or pigments that remove
portions of the visible spectrum.
 The objects may either transmit light
(transparencies) or reflect light (paper, for
example).
 The subtractive primaries are C, M and Y.
 Cyan absorbs red; hence C is sometimes called
"minus red" (-R). Similarly, M is -G and Y is -B.


Colour Models:CMYK
 Cyan-Magenta-Yellow-Key, and pronounced as
separate letters. CMYK
 Based on the subtractive synthesis model
 CMYK is a colour model in which all colours are
described as a mixture of these four process colours.
 CMYK is the standard colour model used in offset
printing for full-colour documents. Because such printing
uses inks of these four basic colours, it is often called
four-colour printing.
 Key is (usually) black – although, in theory, black can be
produced with C+M+Y, it looks muddy and most colour
printers have a separate black ink cartridge.


Colour Models: RGB & CMYK


Matching RBG to CMYK
 One of the most difficult aspects of desktop
publishing in colour is colour matching
 Properly converting the RGB colours into CMYK
colours so that what gets printed looks the same as
what appears on the monitor.
 RGB Most important for multimedia as it corresponds
to the way colour is produced on a computer
monitor

Palettes (Indexed Colour)
 Computer monitors may be able to display 24 bit colour but
video driver hardware may be limited to less than 24 bit.
 Thus we use a subset of 24 bit in an 8 bit palette – a selection
of colours appropriate to the image colours
 Palettes provide a finite sub-set of colours required for a given
image
 If the colour requirements change then some colours in the
palette will need to be substituted for others to enable the new
colours to be represented
 Approximation to original colour in scene


Web Colours
 Websafe colours were specified from the days
when most of the web audience could only
access 256 colours.
 To display intermediate shades, ‘dithering’ was
necessary.
 Most contemporary display hardware is now
able to provide 24 bit colour
 Still need to be aware of potential deployment
issues for now (e.g. mobile phones)


Munsell System
 Albert Henry Munsell
 This system is based on the principle of "perceived
equidistance”
 Munsell attempts to account for each colour attribute
in ordered visual steps. He introduced 100 steps for
colour "hue", starting with five main colours and five
additional colours, and adopts an ordering system
with 10 units of colour "value" and an open scale
called "chroma" (similar to saturation)
 Used in paint industry and photographic applications

RGB -> Pantone conversion
 http://www.netfront.fr/Services/rgb2pa
ntone/pantone.htm?
r=255&g=100&b=100&rgb=

References
The Colour Group ( founded in 1940 ) 19-11-04
http://www.city.ac.uk/colourgroup/
(City University)

Colour Wheel
http://www.ficml.org/jemimap/style/color/wheel.htm

Visbone Colour Resources 19-11-04
http://html-color-codes.com/l

Weinman L. Web Pallets 19-11-04
http://www.lynda.com/

Koren N. Light and Colour
http://www.normankoren.com/light_color.html 21-11-04

Roberts M HSV Colour Model. 21-1104
http://www.cs.bham.ac.uk/~mer/colour/hsv.html

Munsell System
http://www.colorsystem.com/projekte/engl/31mune.htm


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