colorToRGB.js

const colorToRGB = (colorString) => {
    if (colorString.startsWith('rgba(')) {
        const rgba = colorString
            .slice(5, colorString.length - 1)
            .split(',')
            .map((item) => +item.trim());
        const background = [255, 255, 255];
        const rgb = [
            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)
        ];

        return `rgb(${rgb[0]}, ${rgb[1]}, ${rgb[2]})`;
    }

    if (colorString.startsWith('#')) {
        const hex = [colorString.slice(1, 3), colorString.slice(3, 5), colorString.slice(5, 7)].map((item) =>
            parseInt(item, 16)
        );

        return `rgb(${hex[0]}, ${hex[1]}, ${hex[2]})`;
    }

    return colorString;
};