Convert to and from html colors, rgb <-->hsl colors, and darken html colors by a percentage.

color.lua

-- Convert an html color in the format of #RRGGBB into red, green, and blue values on the range of [0..255].
local function hex_to_rgb(html_color)
	html_color = html_color:gsub("#", "")
	return tonumber("0x"..html_color:sub(1, 2)), tonumber("0x"..html_color:sub(3, 4)), tonumber("0x"..html_color:sub(5, 6))
end

-- Convert the red, green, and blue color components back into an html color string.
local function rgb_to_hex(red, green, blue)
	return string.format("#%02X%02X%02X", red, green, blue)
end

-- Convert red, green, and blue values from the range of [0..255] into hue, saturation, and value/lightness values on the range of...?
local function rgb_to_hsv(red, green, blue)
	local r = red / 255
	local g = green / 255
	local b = blue / 255
	local max = math.max(r, g, b)
	local min = math.min(r, g, b)
	local h, s, v = max, max, max

	local d = max - min;
	if max ~= 0 then
		s = d / max
	end

	if max == min then
		h = 0 -- achromatic
	else
		if max == r then
			if g < b then
				h = (g - b) / d + 6
			else
				h = (g - b) / d + 0
			end
		elseif max == g then
			h = (b - r) / d + 2
		elseif max == b then
			h = (r - g) / d + 4
		end
		h = h / 6
	end
	return h, s, v
end

--[[
	Convert the hue, saturation, and value/lightness values output from rgb_to_hsv
	back into red, green, and blue values on the range of [0..255].
]]
local function hsv_to_rgb(hue, saturation, value)
	local i = math.floor(hue * 6)
	local f = hue * 6 - i
	local p = value * (1 - saturation)
	local q = value * (1 - f * saturation)
	local t = value * (1 - (1 - f) * saturation)

	local red, green, blue

	local cases = {
		[0] = function() red, green, blue = value, t, p end,
		[1] = function() red, green, blue = q, value, p end,
		[2] = function() red, green, blue = p, value, t end,
		[3] = function() red, green, blue = p, q, value end,
		[4] = function() red, green, blue = t, p, value end,
		[5] = function() red, green, blue = value, p, q end
	}
	cases[i % 6]()

	return math.floor(red * 255 + 0.5), math.floor(green * 255 + 0.5), math.floor(blue * 255 + 0.5)
end

-- Make the input html color darker by the given percent.
local function darken_color(html_color, percent)
	local r, g, b = hex_to_rgb(html_color)
	local h, s, v = rgb_to_hsv(r, g, b)
	v = v * (1 - percent)
	r, g, b = hsv_to_rgb(h, s, v)
	return rgb_to_hex(r, g, b)
end

I put these functions together for a Minetest mod, but they turned out to not be necessary.