Advanced Colour Functions

You already know lighten(), darken(), fade(), and mix(). Here's the full set, with practical examples for each.

Transparency Functions

alpha(colour, value)

Sets the alpha channel to an exact value. The second argument is a decimal from 0 (fully transparent) to 1 (fully opaque).

yaml

vars:
  overlay: alpha($(accent), 0.5)         # exactly 50% transparent
  solid-accent: alpha($(accent), 1)      # remove any existing transparency
  ghost: alpha($(std.bg), 0.1)           # barely visible

fade(colour, amount)

Reduces opacity by a relative amount. Think of it as "make this more transparent."

yaml

vars:
  std:
    fg.inactive: fade($(std.fg), 60)     # 60% more transparent than fg
    fg.muted: fade($(std.fg), 40)        # 40% more transparent
    outline: fade($(accent), 30)         # subtle accent border

solidify(colour, amount)

The opposite of fade() -- increases opacity by a relative amount.

yaml

vars:
  hover-bg: solidify($(std.bg.accent), 30)  # 30% more opaque

Lightness Functions

lighten(colour, amount) and darken(colour, amount)

Adjust lightness by a percentage. Both work in OKLCH colour space internally for perceptually uniform results.

yaml

vars:
  std:
    bg.panel: lighten($(std.bg), 15)
    bg.panel.inner: lighten($(std.bg.panel), 10)
    bg.deep: darken($(std.bg), 20)

invert(colour)

Flips the lightness value in OKLCH space. A dark colour becomes light, a light colour becomes dark. Hue and chroma are preserved.

yaml

vars:
  inverted-fg: invert($(std.fg))    # light foreground becomes dark
  inverted-bg: invert($(std.bg))    # dark background becomes light

tint(colour, amount) and shade(colour, amount)

Mix toward white or black by a percentage. Useful for quickly generating lighter or darker variants that feel natural rather than washed out.

yaml

vars:
  std:
    bg.hover: tint($(std.bg), 10)      # slightly lighter on hover
    bg.deep: shade($(std.bg), 25)      # noticeably deeper
    accent.soft: tint($(accent), 40)   # pastel-like accent

Chroma Functions

saturate(colour, amount) and desaturate(colour, amount)

Adjust the chroma (colourfulness) of a colour in OKLCH space by a percentage. These are the technical names; see mute and pop below for expressive aliases.

yaml

vars:
  vivid-accent: saturate($(accent), 30)     # push chroma up by 30%
  calm-accent: desaturate($(accent), 40)    # pull chroma down by 40%

grayscale(colour)

Strips all chroma, leaving only the perceptual lightness. The result sits on the same position in the greyscale as the original would appear to the eye.

yaml

vars:
  neutral-bg: grayscale($(std.bg))
  neutral-fg: grayscale($(std.fg))

mute(colour, amount) and pop(colour, amount)

Expressive aliases for partial chroma adjustment. mute moves a colour toward grey; pop moves it away. At 100, mute is equivalent to grayscale.

Think of it as: "I want this colour to feel hushed" vs "I want this to jump out".

yaml

vars:
  std:
    # Active state — accent at full presence
    fg.active: pop($(accent), 20)

    # Inactive states — same hue, much quieter
    fg.inactive: mute($(accent), 60)
    fg.disabled: mute($(accent), 85)

Hue Functions

shiftHue(colour, degrees) and complement(colour)

Rotate the hue angle in OKLCH space. complement is a shorthand for a 180° shift. Both are no-ops on achromatic colours (greyscale), since hue is meaningless there.

yaml

palette:
  base: oklch(0.6 0.15 220)
  triadic-a: shiftHue($$base, 120)
  triadic-b: shiftHue($$base, 240)
  complement: complement($$base)

Contrast

contrast(colour)

Returns #000000 or #ffffff — whichever is more readable against the given colour. Useful for automatically choosing foreground text on dynamic or user-defined backgrounds.

yaml

vars:
  # Automatically readable text on any background
  badge.fg: contrast($(badge.bg))
  button.label: contrast($(button.bg))

Blending

mix(colourA, colourB, ratio)

Blends two colours together. The ratio (0-100) controls the balance -- 0 is all colourA, 100 is all colourB, 50 is an equal mix.

yaml

vars:
  # 20% accent blended into foreground
  tinted-fg: mix($(std.fg), $(accent), 20)

  # Equal blend for a complementary colour
  blend: mix($(colors.blue), $(colors.cyan), 50)

  # Default ratio is 50 if omitted
  halfway: mix($(colors.red), $(colors.green))

Colour Constructors

Any colour expression that Culori understands works as a passthrough. Sassy recognises these as colour constructors and converts them to hex:

yaml

vars:
  # Functional notation
  from-hsl: hsl(210, 60%, 40%)
  from-rgb: rgb(45, 90, 135)
  from-oklch: oklch(0.6 0.15 220)

  # With alpha
  from-hsla: hsla(210, 60%, 40%, 0.8)
  from-rgba: rgba(45, 90, 135, 0.5)
  from-oklcha: oklch(0.6 0.15 220 / 0.7)

  # CSS named colours
  named: css(deepskyblue)
  named-faded: fade(css(crimson), 40)

Colour constructors can also be used inline as arguments to transformation functions -- no variable required:

yaml

vars:
  # Lighten/darken an oklch colour directly
  bg-lighter: lighten(oklch(0.1 0 0), 15)
  accent-dark: darken(hsl(210, 60%, 40%), 20)

  # Fade an rgb colour inline
  subtle: fade(rgb(74, 158, 255), 40)

Building Harmonious Palettes

Combine functions to derive entire palettes from a single base colour. The palette section is ideal for this since it's self-contained:

yaml

palette:
  base: oklch(0.6 0.15 220)
  lighter: lighten($$base, 20)
  darker: darken($$base, 20)
  complement: complement($$base)
  hushed: mute($$base, 60)
  ghost: alpha($$base, 0.15)
  on-base: contrast($$base)

This gives you seven related colours from one source value. Change base and every derived colour follows.

Function Summary

Function	Arguments	Description
lighten	colour, amount	Increase lightness by percentage (OKLCH)
darken	colour, amount	Decrease lightness by percentage (OKLCH)
invert	colour	Flip lightness (OKLCH)
tint	colour, amount?	Mix toward white (default 50%)
shade	colour, amount?	Mix toward black (default 50%)
saturate	colour, amount	Increase chroma by percentage (OKLCH)
desaturate	colour, amount	Decrease chroma by percentage (OKLCH)
grayscale	colour	Remove all chroma
mute	colour, amount	Move toward greyscale by percentage
pop	colour, amount	Move away from greyscale by percentage
shiftHue	colour, degrees	Rotate hue by degrees (OKLCH)
complement	colour	180° hue complement
contrast	colour	#000000 or #ffffff for readability
fade	colour, amount	Reduce opacity (more transparent)
solidify	colour, amount	Increase opacity (more opaque)
alpha	colour, value	Set exact alpha (0--1)
mix	colourA, colourB, ratio?	Blend two colours in OKLCH (ratio 0--100, default 50)
css	name	CSS named colour to hex

Any Culori-supported colour expression (like oklch(...), hsl(...), rgb(...)) also works directly as a value and is converted to hex in the output.

Next Steps

You've got a design system, multiple themes, and a full colour toolkit. Before shipping, let's make sure everything is clean.