颜色通道函数

### hue

Extracts the hue channel of a color object in the HSL color space.
Parameters: `color` - a color object.

Returns: `integer` 0-360

Example: `hue(hsl(90, 100%, 50%))`

Output: `90`

### saturation

Extracts the saturation channel of a color object in the HSL color space.
Parameters: `color` - a color object.

Returns: `percentage` 0-100

Example: `saturation(hsl(90, 100%, 50%))`

Output: `100%`

### lightness

Extracts the lightness channel of a color object in the HSL color space.
Parameters: `color` - a color object.

Returns: `percentage` 0-100

Example: `lightness(hsl(90, 100%, 50%))`

Output: `50%`

### hsvhue

Extracts the hue channel of a color object in the HSV color space.
Parameters: `color` - a color object.

Returns: `integer` 0-360

Example: `hsvhue(hsv(90, 100%, 50%))`

Output: `90`

### hsvsaturation

Extracts the saturation channel of a color object in the HSV color space.
Parameters: `color` - a color object.

Returns: `percentage` 0-100

Example: `hsvsaturation(hsv(90, 100%, 50%))`

Output: `100%`

### hsvvalue

Extracts the value channel of a color object in the HSV color space.
Parameters: `color` - a color object.

Returns: `percentage` 0-100

Example: `hsvvalue(hsv(90, 100%, 50%))`

Output: `50%`

### red

Extracts the red channel of a color object.
Parameters: `color` - a color object.

Returns: `float` 0-255

Example: `red(rgb(10, 20, 30))`

Output: `10`

### green

Extracts the green channel of a color object.
Parameters: `color` - a color object.

Returns: `float` 0-255

Example: `green(rgb(10, 20, 30))`

Output: `20`

### blue

Extracts the blue channel of a color object.
Parameters: `color` - a color object.

Returns: `float` 0-255

Example: `blue(rgb(10, 20, 30))`

Output: `30`

### alpha

Extracts the alpha channel of a color object.
Parameters: `color` - a color object.

Returns: `float` 0-1

Example: `alpha(rgba(10, 20, 30, 0.5))`

Output: `0.5`

### luma

Calculates the luma (perceptual brightness) of a color object.
Uses SMPTE C / Rec. 709 coefficients, as recommended in WCAG 2.0. This calculation is also used in the contrast function.

Before v1.7.0 the luma was calculated without gamma correction, use the luminance function to calculate these "old" values.

Parameters: `color` - a color object.

Returns: `percentage` 0-100%

Example: `luma(rgb(100, 200, 30))`

Output: `44%`

### luminance

Calculates the value of the luma without gamma correction (this function was named luma before v1.7.0)
Parameters: `color` - a color object.

Returns: `percentage` 0-100%

Example: `luminance(rgb(100, 200, 30))`

Output: `65%`