HDR Toggle for Gnome - can be assigned to shortcut and run via gjs. Claude vibe coded so consider this public domain.

hdr-toggle.js

#!/usr/bin/gjs
const { GLib, Gio } = imports.gi;

const BUS_NAME = 'org.gnome.Mutter.DisplayConfig';
const BUS_PATH  = '/org/gnome/Mutter/DisplayConfig';

const ColorMode = { SDR: 0, HDR: 1 };

const connection = Gio.bus_get_sync(Gio.BusType.SESSION, null);

function dbusCall(method, params) {
    return connection.call_sync(
        BUS_NAME, BUS_PATH, BUS_NAME,
        method, params, null,
        Gio.DBusCallFlags.NONE, -1, null
    );
}

function variant(signature, value) {
    return new GLib.Variant(signature, value);
}

function notify(message) {
    Gio.Subprocess.new(
        ['notify-send', '-a', 'HDR Toggle', message],
        Gio.SubprocessFlags.NONE
    );
}

// recursiveUnpack (not deepUnpack) unwraps the per-monitor a{sv} variant containers.
const [serial, monitors, displayLayouts, topLevelProps] =
    dbusCall('GetCurrentState', null).recursiveUnpack();

const monitorState = new Map(monitors.map(([[connector], modes, props]) => {
    // Fall back to first mode when none is marked current — a null mode_id throws opaquely.
    const currentMode = modes.find(m => m[6]['is-current'])?.[0] ?? modes[0]?.[0];
    return [connector, {
        currentMode,
        colorMode:           props['color-mode'] ?? ColorMode.SDR,
        supportedColorModes: props['supported-color-modes'] ?? [ColorMode.SDR],
        underscanning:       'underscanning' in props ? props['underscanning'] : undefined,
    }];
}));

const hdrCapable = [...monitorState.values()].filter(
    m => m.supportedColorModes.includes(ColorMode.HDR)
);

if (!hdrCapable.length) {
    notify('No HDR-capable monitors.');
    imports.system.exit(1);
}

// Treat any non-zero color-mode as "on" so mode 2 (alt HDR variant) isn't toggled to mode 1.
const targetColorMode = hdrCapable.some(m => m.colorMode === ColorMode.SDR)
    ? ColorMode.HDR
    : ColorMode.SDR;

// ApplyMonitorsConfig signature a(iiduba(ssa{sv})) puts props inside each monitor tuple.
const updatedLayouts = displayLayouts.map(([x, y, scale, transform, primary, displays]) =>
    [x, y, scale, transform, primary, displays.map(([connector]) => {
        const m = monitorState.get(connector);
        const props = {};
        if (m.underscanning !== undefined)
            props['underscanning'] = variant('b', m.underscanning);
        if (m.supportedColorModes.includes(ColorMode.HDR))
            props['color-mode'] = variant('u', targetColorMode);
        return [connector, m.currentMode, props];
    })]
);

const applyProps = {};
if (topLevelProps['layout-mode'] !== undefined)
    applyProps['layout-mode'] = variant('u', topLevelProps['layout-mode']);

dbusCall('ApplyMonitorsConfig',
    variant('(uua(iiduba(ssa{sv}))a{sv})', [serial, 2, updatedLayouts, applyProps])
);

notify(`HDR ${targetColorMode === ColorMode.HDR ? 'enabled' : 'disabled'}`);