In this discussion of Google Groups, the default color is not documented and unavailable. However, if you need an array of colors, Xavier Gumara posted it, as well as asgallant :
{"#3366cc","#dc3912","#ff9900","#109618","#990099","#0099c6","#dd4477","#66aa00","#b82e2e","#316395","#994499","#22aa99","#aaaa11","#6633cc","#e67300","#8b0707","#651067","#329262","#5574a6","#3b3eac","#b77322","#16d620","#b91383","#f4359e","#9c5935","#a9c413","#2a778d","#668d1c","#bea413","#0c5922","#743411"}
It’s just FYI, most of these questions were raised in Google groups by people like asgallant, and it can be useful to do a quick search to try to find them (faster than waiting when I do this!). The above answer was the first two results for “default colors for google renderings”.