← ACARA Digital Technologies Years 7–8

RGB values from 0 to 255

Each RGB number runs from 0 (none of that colour) to 255 (full strength). It fits in one byte, which holds 256 values.

Part of: ACARA Digital Technologies Years 7–8 — Representing Text, Images and Sound — Module 33

Understanding RGB values from 0 to 255 in ACARA Digital Technologies Years 7–8

What it does: Each of the three values sits between 0 and 255. Here red is off, green is about half strength and blue is fully on. Eight bits that are all 1 make the largest one-byte number, 255. That is why RGB colour values stop at 255. 0 means none of that colour. A red value of 0 switches the red light off. Each number must be between 0 and 255. 300 is too big. rgb(255, 0, 0) is pure red. Counting starts at 0, so the values run from 0 up to 255. The values 0 to 255 include 0, so there are 256 of them.

Common mistakes: An RGB value can only go from 0 to 255 because it is stored in one byte. 256 is too big, so blue must be a number from 0 to 255, such as 200. Black means every colour is switched off. Green should be 0 (none of that colour), not 255, which is fully on.

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Free printable worksheet with full answer key — or add this module to a class and your students practise it interactively.

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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
What does a red value of 0 mean in an RGB colour?
None of the red light is used (red is off)Red is at full strength
Matches
rgb(300, 0, 0)
↓
300 is too big. RGB values can only go up to 255, so the red value must be 255 or less
Applying
Meanings
Term
rgb(0-255)
Meaning
Each RGB number runs from 0 (none of that colour) to 255 (full strength). It fits in one byte, which holds 256 values.
Practice
<style>
Testing
Gaps
rgb(, 0, 0)
Check Answer
Spots
3green = 128
4blue = 256

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Quick Questions

1. What does a red value of 0 mean in an RGB colour?
A. None of the red light is used (red is off) ✓
B. Red is at full strength
0 means none of that colour. A red value of 0 switches the red light off.
2. What does a green value of 255 mean in an RGB colour?
A. Green is switched off
B. Green is at full strength ✓
255 is the highest value, so that colour is fully on.
3. Which RGB value is NOT allowed for one colour channel?
A. rgb(255, 0, 0)
B. rgb(300, 0, 0) ✓
Each number must be between 0 and 255. 300 is too big. rgb(255, 0, 0) is pure red.
4. What is the range of each number in an RGB colour?
A. 0 to 255 ✓
B. 1 to 256
Counting starts at 0, so the values run from 0 up to 255.
5. Why does each RGB number stop at 255?
A. Each colour uses one byte (8 bits), which holds 256 values from 0 to 255 ✓
B. Computer screens cannot show anything brighter than 255 pixels
One byte is 8 bits. That gives 256 different values, from 0 to 255.

Match the Pairs

rgb(0, 0, 0) → All three colours are off (0), so the pixel is black
rgb(255, 0, 0) → Red is fully on (255) while green and blue are off, so the pixel is pure red
Each RGB number can be any whole number from 0 (off) up to 255 (fully on) → rgb(0, 128, 255)
255 is the top value because each colour is stored in one byte (8 bits), which holds 256 values from 0 to 255 → 11111111 = 255
rgb(300, 0, 0) → 300 is too big. RGB values can only go up to 255, so the red value must be 255 or less
rgb(256, 255, 255) → The first number is one too high. The range is 0 to 255, so 256 is not allowed

Key Term

rgb(0-255)
Each RGB number runs from 0 (none of that colour) to 255 (full strength). It fits in one byte, which holds 256 values.

Fill in the Blanks

1. rgb(255, 0, 0)
Hint: Type the number that makes the red light fully on, the top of the range.
2. rgb(0, 0, 0)
Hint: Type the number that means this colour is completely off, giving black.
3. background: rgb(0, 255, 0);
Hint: Type the largest value one colour can have so the green is at full strength.
4. rgb(255, 255, 255)
Hint: Type the number that turns the blue light fully on, so all three lights make white.
5. top = 2^8 - 1
Hint: One byte holds 256 values, but counting starts at 0. Type the number you take away to get the top value.

Spot the Error

Question 1
A # RGB values are 0 to 255
B red = 255
C green = 128
D blue = 256
Line D — An RGB value can only go from 0 to 255 because it is stored in one byte. 256 is too big, so blue must be a number from 0 to 255, such as 200.
Question 2
A # Black: all three are off
B red = 0
C green = 255
D blue = 0
Line C — Black means every colour is switched off. Green should be 0 (none of that colour), not 255, which is fully on.
Question 3
A p {
B color: rgb(255, 0, 0);
C background: rgb(0, 0, -50);
D }
Line C — RGB numbers cannot be negative. The lowest value is 0, which means none of that colour. It should be rgb(0, 0, 50).
Question 4
A let max = 255;
B let min = -1;
C console.log(min, max);
Line B — The lowest RGB value is 0, which means none of that colour. It can't be -1, so it should be let min = 0;
Question 5
A bits = 4
B values = 2 ** bits
C print(values) # 256
D maxRGB = values - 1
Line A — Each RGB number is stored in one byte, which is 8 bits. With 8 bits there are 256 values (0 to 255), so it should be bits = 8.

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