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              <aside class="sliders">
  <input type="range" id="red" min="0" max="255" value="200" step="1"> 
  <input type="range" id="green" min="0" max="255" value="60" step="1"> 
  <input type="range" id="blue" min="0" max="255" value="255" step="1">

<main class="buttons">  
  <h2>sRBG Luma method</h2>
  <button class="btn">Primary Color</button> 
  <button class="btn btn--secondary">Secondary Color</button> 
  <h2>W3C Luma method</h2>
  <button class="btn btn--w3c">Primary Color</button> 
  <button class="btn btn--w3c btn--secondary">Secondary Color</button>
The challenge: 
1) Set text to either black or white depending on the element background perceived lightness (luma)
2) Set a border as a darker variation of the base color to improve button visibility, ONLY if background luma is really high
3) Automatically generate a secondary, 60º rotated hue color

:root {
  /* theme color variables to use in RGB declarations */
  --red: 200;
  --green: 60;
  --blue: 255;
  /*the threshold at which colors are considered "light". 
Range: decimals from 0 to 1,
recommended 0.5 - 0.6*/
  --threshold: 0.5;
  /*the threshold at which a darker border will be applied.
Range: decimals from 0 to 1,
recommended 0.8+*/
  --border-threshold: 0.8;

.btn {
/*sets the background for the base class*/
	background: rgb(var(--red), var(--green), var(--blue));

Calcs perceived brightness using the 
sRGB Luma method
lightness = (red * 0.2126 + green * 0.7152 + blue * 0.0722) / 255
  --r: calc(var(--red) * 0.2126);
  --g: calc(var(--green) * 0.7152);
  --b: calc(var(--blue) * 0.0722);
  --sum: calc(var(--r) + var(--g) + var(--b));
  --perceived-lightness: calc(var(--sum) / 255);
1) Any lightness value above the threshold will be considered "light", therefore apply a black text color. Any bellow will be considered dark, and use white color. 
This results from appying either a sub-zero (negative) or a higher-than-100 lightness value, which are capped to 0 and 100 respectively, to a HSL declaration
  color: hsl(0, 0%, calc((var(--perceived-lightness) - var(--threshold)) * -10000000%));
2) sets the border as a 50% darker shade of the base color, ONLY if background color luma is higher than the border threshold.
To achieve this I use the same sub-zero or higher-than-max technique, only this time using the Alpha value from an RGBA declaration. 
This results in a border that's either fully transparent or fully opaque
 --border-alpha: calc((var(--perceived-lightness) - var(--border-threshold)) * 100);
  border-width: .2em;
  border-style: solid;
  border-color: rgba(calc(var(--red) - 50), calc(var(--green) - 50), calc(var(--blue) - 50), var(--border-alpha));

/* Alternative calc using the 
W3C luma method
lightness = (red * 0.299 + green * 0.587 + blue * 0.114) / 255
  --r: calc(var(--red) * 0.299);
  --g: calc(var(--green) * 0.587);
  --b: calc(var(--blue) * 0.114);

  /* 3) sets the background color as a 60º rotated hue */

/* just basic styles*/
  background: honeydew;
  max-width: 600px;
  margin: auto;

  grid-template-columns: repeat(auto-fit, minmax(150px,1fr));

  grid-template-columns: repeat(auto-fit, minmax(200px,1fr));
  grid-gap: 1em;

.buttons h2{
  grid-column: 1 / -1;

  padding: 1em;
  font-size: 1.5rem; 
  border-radius: 0.2em;
  box-sizing: border-box;

  color: black;
  margin: 10px;

  content: attr(id);
  text-transform: capitalize;

  counter-reset: red var(--red);
  content: counter(red);

  counter-reset: green var(--green);
  content: counter(green);

  counter-reset: blue var(--blue);
  content: counter(blue);
  JS is used only to set the CSS custom properties used in the base color
  Color changes are calculated in CSS 

const root = document.documentElement;
const inputs = [].slice.call(document.querySelectorAll('input'));

inputs.forEach(input => input.addEventListener('change', handleUpdate));
inputs.forEach(input => input.addEventListener('mousemove', handleUpdate));

function handleUpdate(e) {
  if (this.id === 'red') root.style.setProperty('--red', this.value);
  if (this.id === 'green') root.style.setProperty('--green', this.value);
  if (this.id === 'blue') root.style.setProperty('--blue', this.value);
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