Lecture 04: Color Spaces and JavaScript

COSC 225: Algorithms and Visualization

Spring, 2023


  1. Assignment 01 Grading (input validation)
  2. Assignment 02 Due Tonight
  3. Quiz 02 Wednesday 02/15 (CSS basics)
  4. Assignment 03 Due Friday, 02/15 (JavaScript)
    • mostly uses elements from today’s class


  1. Activity: Flag of Many Colors
  2. Color Spaces
  3. Introducing JavaScript

Last Time: Representing Colors

A color that can be represented on a computer screen is represented by three values:

  1. intensity of red sub-pixel
  2. intensity of green sub-pixel
  3. intensity of blue sub-pixel

Color is a three-dimensional object!

In HTML: rgb(red, green, blue)

  • red, green, blue are integers from 0 to 255.
  • $256^3 \approx 1.7$ million colors!


Manipulation of r, g, b color values is not intuitive

  • red, green, blue have natural physical interpretations
  • combinations of red, green, blue do not have natural perceptual interpretations (at least to me)

Question. What are the RGB values of the color above?

Let’s Make a Rainbow

Used predefined colors:

<div class="flag">
  <div style="background-color: red;" class="stripe"></div>
  <div style="background-color: orange;" class="stripe"></div>
  <div style="background-color: yellow;" class="stripe"></div>
  <div style="background-color: green;" class="stripe"></div>
  <div style="background-color: blue;" class="stripe"></div>
  <div style="background-color: purple;" class="stripe"></div>

The Result

Question. What do you think of HTML’s color choices?

Activity (Pairs)

Make a rainbow with 8 stripes!

  • use RGB colors
  • how to interpolate color values to make rainbow?

Dowload rainbow-eight.html to get started, use RGB color picker


  1. What RGB values did you use for the stripes?
  2. Is there a pattern of how to pick the color of the next stripe?
  3. How do combinations of RGB values relate to your perception of the colors?
    • What adjectives would you use to describe the colors you picked?
  4. Do colors look similar on your screen and the projector?

Colors, Geometry, and Perception

A color is a 3D object: interpret RGB values as coordinates of points in 3D space

Which Colors are “Lighter”?

Which Colors are more “Saturated”?

Where are “Pure” Hues?

Cylindrical View


Perceptual Dimensions:

  • Hue the “pure” color as represented on a rainbow
  • Saturation “intensity” of color
  • Lightness how light (bright) the color appears

Can define mathematical relationship between HSL and RGB coordinates

  • one-to-one correspondance
    • every RGB point has corresponding HSL value
    • every HSL point has corresponding RGB value
  • mathematical relationship between
    • how colors are produced (RGB monitor)
    • how colors are preceived (HSL)



color: rgb(red, green, blue);
  • red, green, blue are integers from 0 to 255


color: hsl(hue, saturation, lightness);
  • hue is a number (degress), nominally from 0 to 359
  • saturation and lightness are percentages (0% to 100%)

HSL Color Picker Demo

Other Color Spaces

There are infinitely many ways to represent colors!

  • RGB and HSL are just two

Others made for differnt hardware/aspects of perception

  • RGB and HSL are “additive” color spaces
  • subtractive spaces, e.g., for paint/dye mixing
    • CMY(K)

Vision Differences

No all people have all three types of color receptors!

  • color blindness affects ~5% of population

Universal design: make graphical that are visually distinctive

  • lightness vs hue/saturation
  • patterns, not just color

Tool: Firefox color vision simulation

  • WebDev Tools -> Accessibility Tab -> Simulate


So Far…

  • HTML specifies document content, structure, semantics
  • CSS specifies display

And now

  • JavaScript specifies interactions

With JavaScript we can

  • create/remove elements
  • modify elements
  • define user interactions


Creating and adding elements to a site!


JavaScript, Two Ways

<!doctype html>
<html lang=en>
    <meta charset=utf-8>
    <title>Page Title</title>
    <script src="hello.js"></script>
	    ...javascript code here...

Take a Look

  • hello.js

Basic Tasks

  • Get an element in the document (selector is like CSS selector)

      const someElement document.querySelector("selector");

    first element in document matching selector is returned

  • Create an element (some-tag is desired tag of element)

      let myElement = document.createElement("some-tag");
  • Add text to element

      myElement.textContent = "some text";
  • Add element as child of another


Adding Style

If someElement is an element, we can…

  • set an id

      someElement.id = "some-id";
  • add a class

  • add a style

      someElement.style.backgroundColor = "rgb(200,200,200)";


Let’s style our example site!

Next Time

  • Visualizing Simple Machines: Cellular Automata!