Color / add an isStringColor utility function

src/ol/color.js

@@ -223,3 +223,14 @@ export function toString(color) {
   const a = color[3] === undefined ? 1 : color[3];
   return 'rgba(' + r + ',' + g + ',' + b + ',' + a + ')';
 }
+
+/**
+ * @param {string} s String.
+ * @return {boolean} Whether the string is actually a valid color
+ */
+export function isStringColor(s) {
+  if (NAMED_COLOR_RE_.test(s)) {
+    s = fromNamed(s);
+  }
+  return HEX_COLOR_RE_.test(s) || s.indexOf('rgba(') === 0 || s.indexOf('rgb(') === 0;
+}