Colour science

Additive and subtractive mixing, the RGB, CMYK and hue-saturation-lightness models, hexadecimal colour codes, luminance and greyscale, gamut.

The outline, topic by topic

Each topic is weighted the way the exam weighs it.

RGB model: what is it?
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A description of a colour by its amounts of red, green and blue, each from 0 to 255 in 8 bits.
It is the model of screens: (255, 0, 0) is pure red, (0, 0, 0) black, (255, 255, 255) white.
Which colour does the code #00FF00 give?
- A pure green: green at maximum, the other two channels off
- A magenta: green at zero, red and blue at full
- An off-white: all three channels on at half, with no hue
- A blue-green: the green and blue channels both fully on
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A pure green: green at maximum, the other two channels off
The pairs are read in the order red, green, blue: 00, FF, 00. Magenta, which inverts each channel, would be #FF00FF; the blue-green (cyan) would be #00FFFF.
The … of a colour measures its perceived brightness; since the eye is more sensitive to … than to blue, green weighs the most in it.
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The luminance of a colour measures its perceived brightness; since the eye is more sensitive to green than to blue, green weighs the most in it.
To convert to greyscale, a plain average of the three channels gives a wrong result: the formula 0.299 R + 0.587 G + 0.114 B respects the eye's sensitivity.
CMYK model: what is it?
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A description of a colour by its amounts of cyan, magenta, yellow and black ink, used in printing.
Black is added because mixing the other three inks gives a dark brown rather than a true black, and to save ink.