Avoid creating too-similar VM labels

[marmarek]

If we in practice want to allow creating custom labels, it would be nice to avoid them being too similar.
Some PoC:

QubesOS/qubes-core-admin#95 (comment)

import math

GAMMA = 2.2
#THRESHOLD = 

def hex_to_float(color, gamma=1.0):
  return tuple((int(color[i:i+2], 0x10) / 255.0) ** gamma for i in range(0, len(color), 2))

untrusted_r, untrusted_g, untrusted_b = hex_to_float(untrusted_color, gamma=GAMMA)
for label in existing_labels:
  r, g, b = hex_to_float(label.color, gamma=GAMMA)
  # approximate the colour distance as Euclidean distance in linear (gamma-expanded) space
  if math.sqrt((r - untrusted_r) ** 2 + (g - untrusted_g) ** 2 + (b - untrusted_b) ** 2) < THRESHOLD:
    raise qubes.exc.QubesValueError('colour too similar to an existing one')