See the icons you can get thanks to the demo page :
Icon Service v2 - MapQuest API Documentation
Get flags or map-markers :
The plugin works without specifying a key, but you can create a free account if you think you'll get an intensive usage of it.
Create a Free account - up to 15 000 API call /month
MapQuest documentation is pretty clear :
To set the color, indicate the hexacode. No need to add "#" as a prefix.
Some icons , symbols can be used.
Icon sizes :
To get a higher resolution, use the retina option adding "@2x".
Icon types :