← ACARA Digital Technologies Years 7–8

The RGB colour model

A colour model where any screen colour is made by mixing red, green and blue light, written as three numbers (R, G, B). Each pixel on a screen uses its own three numbers.

Part of: ACARA Digital Technologies Years 7–8 — Representing Text, Images and Sound — Module 32

Understanding The RGB colour model in ACARA Digital Technologies Years 7–8

What it does: The RGB model writes a colour as three numbers in the order red, green, blue. Red and green are both at 255 and blue is 0, so the mix of red and green light looks yellow. Every pixel stores its own three numbers, which is how different pixels can show different colours. RGB is about light from a screen, not paint. Light mixing is different from mixing paints. Computers store numbers, so a pixel's colour is kept as its (R, G, B) numbers instead of a name. The fix is to add the blue number, such as (255, 0, 0). Each pixel needs all three values.

Common mistakes: An RGB colour needs three numbers: red, green and blue. This pixel has only two. It should be (0, 255, 0). RGB is written in the order (R, G, B). (255, 0, 0) is full red. Pure blue puts 255 in the third position: (0, 0, 255).

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This module's key terms are also in a printable crossword (Modules 128–152) and word search (Modules 69–152).

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Learning
Duo
Which three colours of light does the RGB colour model mix to make screen colours?
Red, green and blueRed, yellow and blue
Matches
(255, 0)
↓
An RGB colour needs three numbers, one each for red, green and blue. This one only has two, so the blue value is missing.
Applying
Meanings
Term
RGB
Meaning
A colour model where any screen colour is made by mixing red, green and blue light, written as three numbers (R, G, B). Each pixel on a screen uses its own three numbers.
Practice
<div style="background: rgb(255, 0, 0)">Red</div>
Testing
Gaps
(255, 0, 0)
Check Answer
Spots
1# A pure green pixel
2pixel = (0, 255)

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Quick Questions

1. Which three colours of light does the RGB colour model mix to make screen colours?
A. Red, yellow and blue
B. Red, green and blue ✓
RGB stands for Red, Green, Blue. Screens mix these three colours of light to make every other colour.
2. How many numbers does the RGB model use to describe one colour?
A. Three: one each for red, green and blue ✓
B. One: a single number for the whole colour
An RGB colour is a set of three numbers, one for each light colour.
3. Which one is a colour written in RGB form?
A. (255, 0, 0) ✓
B. (255, 0)
RGB needs three numbers in the order (R, G, B). (255, 0, 0) is full red with no green or blue.
4. In the RGB colour (R, G, B), which number comes first?
A. The amount of red ✓
B. The amount of blue
The order is always Red, Green, Blue, just like the letters R, G, B.
5. On a screen, each pixel has its own RGB numbers.
A. False
B. True ✓
Every pixel stores its own three numbers, which is how different pixels can show different colours.

Match the Pairs

(255, 0, 0) → An RGB colour with full red and no green or blue, so the pixel shows bright red.
(0, 0, 255) → An RGB colour with no red or green and full blue, so the pixel shows bright blue.
Every colour on a screen is a mix of red, green and blue light, so it needs three numbers in the order R, G, B. → (R, G, B)
Mixing full red and full green light, with no blue, makes yellow on a screen. → (255, 255, 0)
(255, 0) → An RGB colour needs three numbers, one each for red, green and blue. This one only has two, so the blue value is missing.
(0, 0, 255) is red → The order is R, G, B, so the third number is blue, not red. (0, 0, 255) is blue. Red would be (255, 0, 0).

Key Term

RGB
A colour model where any screen colour is made by mixing red, green and blue light, written as three numbers (R, G, B). Each pixel on a screen uses its own three numbers.

Fill in the Blanks

1. rgb(255, 0, 0)
Hint: Type the three-letter name of the colour model that mixes red, green and blue light.
2. rgb(255, 0, 0)
Hint: Type the highest value the red number can have, so red light is fully on.
3. rgb(255, 255, 0)
Hint: Yellow uses no blue light. Type the number that means blue is switched off.
4. color: rgb(255, 255, 255);
Hint: White needs every colour at full strength. Type the red value for white.
5. let red = 255; let green = 128; let blue = 0;
Hint: Type the name of the middle colour channel in (R, G, B).

Spot the Error

Question 1
A # A pure green pixel
B pixel = (0, 255)
C print(pixel)
Line B — An RGB colour needs three numbers: red, green and blue. This pixel has only two. It should be (0, 255, 0).
Question 2
A # A pure blue pixel
B pixel = (255, 0, 0)
C print(pixel)
Line B — RGB is written in the order (R, G, B). (255, 0, 0) is full red. Pure blue puts 255 in the third position: (0, 0, 255).
Question 3
A pixel = (255, 0, 255)
B # Red + green light is yellow
C print(pixel)
Line A — Yellow is made by mixing full red and full green light with no blue. That is (255, 255, 0). (255, 0, 255) is red plus blue, which gives magenta.
Question 4
A red = 255
B green = 165
C blue = 0
D pixel = (red, blue, green)
E print(pixel)
Line D — The order of an RGB colour is always red, green, blue. Swapping green and blue changes the colour. It should be (red, green, blue).
Question 5
A h1 {
B color: rgb(255, 0, 0);
C background: rgb[0, 0, 0];
D }
Line C — CSS writes the three RGB numbers inside round brackets, not square brackets. It should be rgb(0, 0, 0).

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