colorToHEX.js

const colorToHEX = (colorString) => {
    if (colorString.startsWith('rgba(')) {
        const rgba = colorString
            .slice(5, colorString.length - 1)
            .split(',')
            .map((item) => +item.trim());
        const background = [255, 255, 255];
        const hex = [
            Math.round((rgba[3] * (rgba[0] / 255) + (1 - rgba[3]) * (background[0] / 255)) * 255),
            Math.round((rgba[3] * (rgba[1] / 255) + (1 - rgba[3]) * (background[1] / 255)) * 255),
            Math.round((rgba[3] * (rgba[2] / 255) + (1 - rgba[3]) * (background[2] / 255)) * 255)
        ].map((item) => `0${item.toString(16)}`.slice(-2));

        return `#${hex[0]}${hex[1]}${hex[2]}`;
    }

    if (colorString.startsWith('rgb(')) {
        const rgb = colorString
            .slice(4, colorString.length - 1)
            .split(',')
            .map((item) => +item.trim());
        const hex = rgb.map((item) => `0${item.toString(16)}`.slice(-2));

        return `#${hex[0]}${hex[1]}${hex[2]}`;
    }

    return colorString;
};