Calculate luma (brightness) of a hexadecimal colour

hex2luma.js

// Calculate luma (brightness) of a hexadecimal colour
// ---------------------------------------------------
// You can use this to calculate whether two colours
// are suitable to be used as a background and text

function hex2luma(hex) {
  return (
    (parseInt(hex.substring(0, 2), 16) * 299) + 
    (parseInt(hex.substring(2, 4), 16) * 587) + 
    (parseInt(hex.substring(4, 6), 16) * 114)
  ) / 1000;
}