Colouring the web

How do you define a colour for a machine?

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When building stuff for the web, it’s likely you’ll need to inject a little bit of colour into your project at some point.

But, how do we describe a colour to a machine?

In the web world at least we can express colour in several different formats. The main ones we’ll look at today are Hex, RGB and HSL.

Hex (hexadecimal)

A hex code is made from a six-digit (sometimes three digits, we’ll get to that), three-byte hexadecimal number.

Each byte represents the red, green and blue components of a colour.

Now, that all sounds a bit nerdy and this is supposed to be about the visual medium of colour so, let’s visualise.

The hex code for this green-ish colour is my favourite hex code, #bada55. To use it in CSS we’d write something like this:

.thing {
	color: #bada55;

# - Tells the browser we’re using a hex.

ba - Red component.

da - Green component.

55 - Blue component.

What are these letters and numbers?! How does ba mean ‘red’?!

Probably a post for another day, but briefly put, hexadecimal or base 16, uses 16 symbols to represent a number (0-9 and A-F, where the latter represents 10-15). We, humans, favour the simpler base 10 (0-9), decimal system, thank goodness.

We’ll use parseInt in JavaScript to humanise the value:

parseInt('ba', 16);
// → 186

Here we are asking for a string with a value of "ba" to be parsed as an integer. To help JavaScript know what it’s dealing with, we provide a radix (the base) of 16 to tell it to expect base 16.

HEX Visualiser

Use the sliders below to generate a HEX colour.



You may notice that when all the colour components are the same, we get a shade of grey.

Hex shorthand

Hex codes can also be provided as three digits if each colour component has two of the same digit.

For example #ff99cc could be written as #f9c.

In a time, long ago, before CSS variables and CSS-in-JS, where you’d be writing the same hex code over and over, optimisations like this could help reduce the size of your files. However, most pre/post-processing systems will make this optimisation on your behalf, when minifying.


You can optionally provide a fourth component to a hex that represents the alpha channel.

For example, #bada5580.

As with the colours, this is provided as a hex. 0 is transparent and 1 is opaque. 0.5 is semi-transparent.

RGB (Red, Green, Blue)

Following on from the above, if we convert our hex code to RGB we get a nicer decimal value.

Here’s a converter that does just that.

Hex to RGB converter

Select a colour using the colour picker and watch the RGB values change.

  • Red: 186 (73%)
  • Green: 218 (85%)
  • Blue: 85 (33%)

These values are between 0-255, where 0 is 0% and 255 is 100% of each colour component.

We can use an RGB value in CSS with:

.thing {
	color: rgb(186, 218, 85);

Much like the hex, as before:

rgb tells the browser we’re using RGB and to use the values between the parenthesise.

186 is the red component.

218 is the green component.

85 is the blue component.

If red, green and blue are all 255 (100%) what would you expect? Well, having two kids who like to use all the paints 🙈, my expectation would be a murky brown! This however is not what we get, in fact rgb(255, 255, 255) gives us white and rgb(0, 0, 0), black.

What! Yep, I know, madness.

Think of it like this - imagine you’re in a completely dark room and you have three torches, one for each colour component (red, green and blue) and these torches are all off (set to 0). What colour would the room be? Well, your torches are off, the room is black and void of light.

Now let’s turn your torches on (set to 255) and shine them all together at a single point - you’d get white! Weird, but, SCIENCE.

RGB/RGBA Visualiser

Use the sliders below to generate an RGB(A) colour.

rgb(186, 218, 85)


As with hex, when all the digits are the same we get a shade of grey.


To use alpha you need to switch out rgb for rgba and provide a fourth argument.

For example, rgba(186, 218, 85, 0.5).

0 is transparent and 1 is opaque. 0.5 is semi-transparent.

HSL (Hue, Saturation, Lightness)

Compared to Hex and RGB, which focus on the mixing of the three core colours, HSL is a little more complicated. It does, however, better align with how we perceive colour-making.

Hue is a degree on the colour wheel from 0-360. 0 = Red, 120 = Green and 240 = Blue.

Saturation is a percentage, where 0% means a shade of grey, 100% being the full colour.

Lightness is a percentage, where 0% is black and 100% is white.

HSL/HSLA Visualiser

Use the sliders below to generate an HSL(A) colour.

hsl(74, 64.3%, 59.4%)

As with RGB, we use a similar syntax to tell the browser what do show.

hsl tells the browser we’re using HSL and to use the values between the parenthesise.

74 is the hue (degrees).

64% is the saturation.

59% is the lightness.


Unlike hex and RGB, HSL becomes achromatic when you drop saturation down to 0. You can then use lightness to change the shade.


To use alpha you need to switch out hsl for hsla and provide a fourth argument.

For example, hsla(74, 64%, 59%, 0.5).

0 is transparent and 1 is opaque. 0.5 is semi-transparent.

What should I use?

Personally I lean towards using Hex codes for fixed colours, but more to rgb and hsl when needing to have more explicit control or power to manipulate the colour. hsl, for example, can play really nicely with theming where you might want to save the hue and saturation to a CSS custom property and alter the lightness based on context.

For example:

:root {
	--hs: 74, 64%;

body {
	background-color: hsl(var(--hs), 90%);

a {
	color: hsl(var(--hs), 30%);

Next up

In my next post, we’ll take a look at programmatically manipulating colours and how we might interpolate between them.