Hex to CMYK

These examples use the same algebraic channel mapping, with input validation. CMYK results keep full precision; round to one decimal for display. No ICC profile is applied.

JavaScript: RGB to CMYK

function rgbToCmyk(r, g, b) {
  if (![r, g, b].every(v => Number.isInteger(v) && v >= 0 && v <= 255)) {
    throw new RangeError("RGB channels must be integers from 0 to 255");
  }
  const x = Math.max(r, g, b);
  return x === 0 ? [0, 0, 0, 100] : [
    100 * (x - r) / x, 100 * (x - g) / x,
    100 * (x - b) / x, 100 * (1 - x / 255)
  ];
}

Python: RGB to CMYK and CMYK to RGB

import math

def rgb_to_cmyk(r, g, b):
    if any(type(v) is not int or not 0 <= v <= 255 for v in (r, g, b)):
        raise ValueError("RGB channels must be integers from 0 to 255")
    x = max(r, g, b)
    return [0, 0, 0, 100] if x == 0 else [
        100 * (x-r)/x, 100 * (x-g)/x,
        100 * (x-b)/x, 100 * (1-x/255)
    ]

def cmyk_to_rgb(c, m, y, k):
    if any(type(v) not in (int, float) or not math.isfinite(v)
           or not 0 <= v <= 100 for v in (c, m, y, k)):
        raise ValueError("CMYK percentages must be finite and from 0 to 100")
    return [math.floor(255 * (1-v/100) * (1-k/100) + 0.5)
            for v in (c, m, y)]