Expand docs for color options

src/ol/color/color.js

@@ -110,7 +110,7 @@ ol.color.asArray = function(color) {
 
 /**
  * @param {ol.Color|string} color Color.
- * @return {string} String.
+ * @return {string} Rgba string.
  * @api
  */
 ol.color.asString = function(color) {

src/ol/color/color.jsdoc

@@ -1,3 +1,8 @@
 /**
+ * Colors can be defined as a {@link ol.Color} array, or as strings in
+ * `rgb(r,g,b)` or `rgba(r,g,b,a)` format, or in hex `#rrggbb` or `#rgb` format.
+ * Color names, like 'red', 'blue' or 'green', may also be used with the
+ * Canvas renderer.
+ *
  * @namespace ol.color
  */