A simple Stylus function for getting the contrasting text color (black or white) based on the background color. Extracted from Inkline framework https://github.com/inkline/inkline.

contrastColor.styl

/**
 * Get contrast text color for a background color by calculating the luminance
 * of the given input color.
 *
 * @param color Text background color
 * @return Contrasting text color
 */
contrastColor(color)
  RGB = {
    r: red(color)
    g: green(color)
    b: blue(color)
  }

  for k, c in RGB
    c = c / 255.0
    if c <= 0.03928
      c = c / 12.92
    else
      c = ((c + 0.055) / 1.055) ** 2.4
    RGB[k] = c


  L = 0.2126 * RGB.r + 0.7152 * RGB.g + 0.0722 * RGB.b


  if L > 0.179
    return #000000
  else
    return #ffffff