Lecture 03: Color and Style

COSC 225: Algorithms and Visualization

Spring, 2023


  1. Assignment 02 Due Monday, 02/13
  2. Quiz 02 Wednesday 02/15 (CSS basics)
  3. Assignment 03 Due Friday, 02/15 (short)


  1. CSS Crash Course
  2. Color

Last Time

Colored Boxes!

To Draw a Box

  • use div elements
  • use style attribute to specify
    • dimensions (width, height)
    • positioning system (position: static, relative, absolute)
    • position coordinates (top, left)
    • color (background-color)
    <div style="background-color: black; width: 300px; 
	height: 300px; position: relative;">
      <div style="background-color: blue; width: 100px; 
	  height: 100px; top: 50px; left: 50px; position: 


Writing style = "..." for every element is cumbersome!

  • a lot of text to draw a single box
  • making changes is a pain
  • many boxes may have similar style attributes
  • no semantics associated with style attribute values

Setting style manually is not good style!

… if only there was a better way…

Introducing CSS

Cascading Style Sheets (CSS): specify style attributes for many elements on a page:

  • all elements with same tag (e.g., h1, p, div)
  • all elements of same class (set attribute class="some-class")
  • an element with a specific id (set attribute id="some-id")

Change to CSS styles affect all elements matching a prescribed pattern

Boxes Revisited

Example from before

  • big outer box
  • smaller boxes inside

Updating the HTML

Rather than setting style attribute directly, specify semantics

  • use class attribute to give names to the types of boxes
    <div class="outer-box">
      <div class="inner-box"></div>
      <div class="inner-box"></div>

Pick Style for Boxes by Type

All boxes (div elements):

  • position: absolute;

Outer box:

  • width: 300px;
  • height: 300px;
  • background-color: black;

Inner boxes:

  • width: 100px;
  • height: 100px;

Where to Put CSS Styling?

Two methods:

  1. In .html head:

            /* style goes here */
  2. A separate file, say style.css, then add reference in head:

         <link rel="stylesheet" href="style.css">

CSS Style

  • apply style to all div elements in the document
      div {
        position: absolute;
  • apply style to all elements with class="outer-box"
      .outer-box {
        background-color: black;
        width: 300px;
        height: 300px;

Dealing with Inner Boxes

  • apply style to all elements with class="inner-box"
      .inner-box {
        width: 100px;
        height: 100px;

Coloring Inner Boxes?

Each inner box has own:

  • position (top, left)
  • color (background-color)

CSS can style element by id!

Give inner boxes unique ids:

      <div id="blue-box" class="inner-box"></div>
      <div id="red-box" class="inner-box"></div>

Style Elements by ID

#blue-box {
    background-color: blue;
    top: 50px;
    left: 50px;

#red-box {
    background-color: red;
    top: 150px;
    left: 100px;




  1. CSS properties can conflict
    • more specific rules win
      • id beats class beats tag
    • if same specificity, last rule in css source wins
        p { color: red; }
        p { color: blue;}
        <p>What color is this text</p>
  2. Elements can have multiple classes, but id should be unique

       <div id="red-box" class="inner-box special-box"></div>

Assignment 02

Go forth and make a stylish website!


Color and Perception

What is Color?

Color, Three Ways:

  1. subjective perception of color
  2. physical production of color
  3. formal representation of color

Content Warning: Massive oversimplifications coming up!!!

Color and Light

Physics $\implies$ Perception

  • Color perception begins with light
    • light enters the eye
    • light stimulates receptors in the retina
    • retinal stimulation results in perception (somehow)
  • “Pure” light has two attributes
    • wavelength: hue (e.g., blue, green, red)
    • intensity: brightness

image source: Wikipedia

Perception of Pure Light

  • “Trichromatic” humans have three types of color receptors (cones) in their retina
  • each receptor has characteristic sensitivity to different wavelengths

image source: Feynman Lectures on Physics

Natural Light

“Natural” light comprised of different wavelengths in different proportions

image source: Wikipedia

Perception of Color

Perception of color determined by the amount each color receptor is stimulated

  • many different light power spectra correspond indistinguishable colors
  • $\implies$ it is possible represent many colors by “mixing” a fixed set of colors

Generation of Color

Question. How do color monitors/projectors create so many colors?

  • Display is a 2d grid of pixels
  • Each pixel contains multiple (3?) light producing elements
    • red
    • green
    • blue
  • Intensities of each element can be controlled independently

Different Types of Displays

Engineering $\implies$ Perception

So far:

  • can generate light with different characterstics:
    • vary intensity (brightness) of three different pixel elements
    • red, green, blue
  • light emitted by pixels stimulates retina
    • red pixel light stimulates red cones more
  • relative stimulation of different cones in retna $\implies$ perception of different colors

Formal Representation of Color

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!

Color Picker Demo


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 so flag looks “rainbowish?”

Dowload rainbow-eight.html to get started


  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?

Next Time

  1. Colors + Geometry = Color Spaces
  2. Introducing JavaScript