Print out color pairs with their contrast

colors.json

[
  {
    "type": "colordef",
    "BLACK": [ 33, 34, 37 ],
    "RED": [ 254, 63, 22 ],
    "GREEN": [ 60, 168, 103 ],
    "BROWN": [ 223, 112, 30 ],
    "BLUE": [ 0, 106, 170 ],
    "MAGENTA": [ 235, 82, 177 ],
    "CYAN": [ 7, 163, 167 ],
    "DGRAY": [ 138, 139, 172 ],
    "LRED": [ 255, 158, 138 ],
    "LGREEN": [ 152, 206, 119 ],
    "YELLOW": [ 212, 182, 52 ],
    "LBLUE": [ 30, 171, 255 ],
    "LMAGENTA": [ 247, 156, 188 ],
    "LCYAN": [ 58, 206, 192 ],
    "GRAY": [ 184, 186, 188 ],
    "WHITE": [ 255, 255, 255 ]
  }
]

contrast.py

#!/usr/bin/env python3

import os, sys, argparse, json


def translate(value, value_min_range, value_max_range, min_range, max_range):
    value_span = value_max_range - value_min_range
    span = max_range - min_range
    scaled = float(value - value_min_range) / float(value_span)
    return min_range + (scaled * span)


def rgb_as_int(rgb1, rgb2):
    n_rgb1 = tuple([translate(c, 0, 255, 0, 1) for c in rgb1])
    n_rgb2 = tuple([translate(c, 0, 255, 0, 1) for c in rgb2])
    return rgb(n_rgb1, n_rgb2)


# https://www.w3.org/TR/WCAG21/#dfn-contrast-ratio
def rgb(rgb1, rgb2):
    for r, g, b in (rgb1, rgb2):
        if not 0.0 <= r <= 1.0:
            raise ValueError("r is out of valid range (0.0 - 1.0)")
        if not 0.0 <= g <= 1.0:
            raise ValueError("g is out of valid range (0.0 - 1.0)")
        if not 0.0 <= b <= 1.0:
            raise ValueError("b is out of valid range (0.0 - 1.0)")

    l1 = _relative_luminance(*rgb1)
    l2 = _relative_luminance(*rgb2)

    if l1 > l2:
        contrast = (l1 + 0.05) / (l2 + 0.05)
    else:
        contrast = (l2 + 0.05) / (l1 + 0.05)

    if contrast >= 7.0:
        return "AAA"
    elif contrast >= 4.5:
        return "AA"
    else:
        return round(contrast, 3)


# https://www.w3.org/TR/WCAG21/#dfn-relative-luminance
def _relative_luminance(r, g, b):
    r = _linearize(r)
    g = _linearize(g)
    b = _linearize(b)

    return 0.2126 * r + 0.7152 * g + 0.0722 * b


def _linearize(v):
    if v <= 0.03928:
        return v / 12.92
    else:
        return ((v + 0.055) / 1.055) ** 2.4


if __name__ == "__main__":
    parser = argparse.ArgumentParser()
    parser.add_argument(
        "-j",
        "--colors",
        dest="base_colors",
        type=str,
        help="path to base colors json",
        default="",
    )
    parser.add_argument(
        "-p",
        "--print",
        dest="print",
        type=str,
        help="print out color pairs, you'll need terminal with 24-bit color support enabled; enabled by default",
        default="yes",
        choices=["yes", "no"],
    )
    args = parser.parse_args(args=None if sys.argv[1:] else ["--help"])

    with open(args.base_colors, "rt") as f:
        bc_json = json.load(f)[0]

    colors = []
    column_width = 0
    for record in bc_json:
        if record != "type":
            column_width = max(column_width, len(record) + 2)
            pair = {
                "name": record,
                "rgb": (bc_json[record][0], bc_json[record][1], bc_json[record][2]),
            }
            colors.append(pair)

    for c1 in colors:
        for c2 in colors:
            if c1["name"] != c2["name"]:
                if args.print == "yes":
                    print(
                        f'{c1["name"]:>{column_width}s} {c2["name"]:>{column_width}s} {rgb_as_int(c1["rgb"], c2["rgb"]):>7} \x1b[48;2;{c1["rgb"][0]};{c1["rgb"][1]};{c1["rgb"][2]}m\x1b[38;2;{c2["rgb"][0]};{c2["rgb"][1]};{c2["rgb"][2]}m {c2["rgb"][0]:3} {c2["rgb"][1]:3} {c2["rgb"][2]:3} \x1b[0m'
                    )
                else:
                    print(
                        f'{c1["name"]:>{column_width}s} {c2["name"]:>{column_width}s} {rgb_as_int(c1["rgb"], c2["rgb"]):>7}'
                    )

contrast_apca.py

#!/usr/bin/env python3

import os, sys, argparse, json


## https://github.com/Myndex/SAPC-APCA/blob/6ef8c9982973ac4514fa69c717b838d9c0093251/documentation/APCA-W3-LaTeX.md
## direct contrast, without font and score interpolation
def lightness_contrast(rgb1, rgb2):
    s_apc = _lightness_difference(*rgb1, *rgb2)
    w_offset = 0.027
    w_clamp = 0.1

    if abs(s_apc) < w_clamp:
        return 0

    if s_apc > 0:
        return abs(round((s_apc - w_offset) * 100.0, 1))
    else:
        return abs(round((s_apc + w_offset) * 100.0, 1))


def _lightness_difference(r1, g1, b1, r2, g2, b2):
    w_scale = 1.14
    d_min = 0.0005
    n_tx = 0.57
    n_bg = 0.56
    r_tx = 0.62
    r_bg = 0.65

    y_bg = _luminance(r1, g1, b1)
    y_tx = _luminance(r2, g2, b2)

    if abs(y_bg - y_tx) < d_min:
        return 0

    if y_bg > y_tx:
        return (y_bg**n_bg - y_tx**n_tx) * w_scale
    else:
        return (y_bg**r_bg - y_tx**r_tx) * w_scale


def _luminance(r, g, b):
    s_trc = 2.4
    b_thrsh = 0.022
    b_clip = 1.414

    y_r = ((r / 255.0) ** s_trc) * 0.2126729
    y_g = ((g / 255.0) ** s_trc) * 0.7151522
    y_b = ((b / 255.0) ** s_trc) * 0.0721750
    y = y_r + y_g + y_b

    if y < b_thrsh:
        return y + ((b_thrsh - y) ** b_clip)
    else:
        return y


if __name__ == "__main__":
    parser = argparse.ArgumentParser()
    parser.add_argument(
        "-j",
        "--colors",
        dest="base_colors",
        type=str,
        help="path to json",
        default="",
    )
    parser.add_argument(
        "-p",
        "--print",
        dest="print",
        type=str,
        help="print out color pairs",
        default="yes",
        choices=["yes", "no"],
    )
    args = parser.parse_args(args=None if sys.argv[1:] else ["--help"])

    with open(args.base_colors, "rt") as f:
        bc_json = json.load(f)[0]

    colors = []
    column_width = 0
    for record in bc_json:
        if record != "type":
            column_width = max(column_width, len(record) + 2)
            pair = {
                "name": record,
                "rgb": (bc_json[record][0], bc_json[record][1], bc_json[record][2]),
            }
            colors.append(pair)

    for c1 in colors:
        for c2 in colors:
            if c1["name"] != c2["name"]:
                if args.print == "yes":
                    print(
                        f'{c1["name"]:>{column_width}s} {c2["name"]:>{column_width}s} {lightness_contrast(c1["rgb"], c2["rgb"]):>6} \x1b[48;2;{c1["rgb"][0]};{c1["rgb"][1]};{c1["rgb"][2]}m\x1b[38;2;{c2["rgb"][0]};{c2["rgb"][1]};{c2["rgb"][2]}m {c2["rgb"][0]:3} {c2["rgb"][1]:3} {c2["rgb"][2]:3} \x1b[0m'
                    )
                else:
                    print(
                        f'{c1["name"]:>{column_width}s} {c2["name"]:>{column_width}s} {lightness_contrast(c1["rgb"], c2["rgb"]):>6}'
                    )

        small body text only
> 89.7
        body text okay
> 74.7 
        fluent text okay
> 59.7 
        large & sub-fluent text
> 44.7 
        spot & non text only
> 29.7 
        no text usage
> 14.7 
        invisible