rgb-progress-bar/style.css

.ytp-play-progress,
.ytp-swatch-background-color {
    animation: RGB 25s ease-in-out infinite;
}

@keyframes RGB {
    from {
        background-color: #FF0000;
    }

    7% {
        background-color: #FF7700;
    }

    14% {
        background-color: #FFDD00;
    }

    21% {
        background-color: #00FF00;
    }

    28% {
        background-color: #0000FF;
    }

    35% {
        background-color: #8A2BE2;
    }

    42% {
        background-color: #C77DF3;
    }

    49% {
        background-color: #C77DF3;
    }

    56% {
        background-color: #8A2BE2;
    }

    63% {
        background-color: #0000FF;
    }

    70% {
        background-color: #00FF00;
    }

    77% {
        background-color: #FFDD00;
    }

    84% {
        background-color: #FF7700;
    }

    91% {
        background-color: #FF0000;
    }
}