var html = @"<p style='color: rgb(255 122 127 / 80%)'>Text</p>";
var parser = new HtmlParser(new HtmlParserOptions(), BrowsingContext.New(Configuration.Default.WithCss(new CssParserOptions())));
var dom = parser.ParseDocument(html);
var p = dom.QuerySelector("p");
var s = p.GetStyle();
var color = s.GetColor(); // -> ""
rgb() on MDN
rgb()on MDN