lightColor algorithm produces incorrect colors for specific sources

[arnemolland]

While the algorithm produces palettes as expected for most source colors, but produces inaccurate light colors for some sources. This has been known for some time, but we haven't had any incentive as we're not producing colors in the error-prone color space in our own internal systems.

Some known error-prone source colors:

Source	Expected	Produced
#11aa11	#~adf1aa	#373c74

Without having dug too deep, we have 3 possible sources of error:

• HSVColor from package:flutter/src/painting/colors.dart
• BrightenExtension:
  

  flume_mobile/lib/src/ambiance/algorithm/brighten.dart

  Lines 11 to 17 in c27db34

  	Color darken(double amount) {
  	if (amount == 0) return this;
  	final lab = CIELAB.fromColor(this);
  	var l = lab.l;
  	l -= kn * amount;
  	return CIELAB(l, lab.a, lab.b).toColor();
  	}

• SaturateExtension:
  

  flume_mobile/lib/src/ambiance/algorithm/saturate.dart

  Lines 9 to 16 in c27db34

  	Color saturate(double amount) {
  	final lch = LCH.fromColor(this);
  	var c = lch.c;
  	c += amount * kn;
  	if (c < 0) c = 0;
  	return LCH(lch.l, c, lch.h).toColor();
  	}

Where my initial bet is my implementations of CIELAB or LCH conversions being incorrect.

What needs to be done:

• Debug and find the source of error. The [chroma.js)[https://gka.github.io/chroma.js/] and our web-based preview might be helpful in the process, as they're proven to be correct. We don't need and we probably won't get a 100% match due to language details and float precision, but we want to get close.
• Change the implementation where needed
• Update tests
• Possibly increase amount of test cases as we go, depends on investigations