Well it looks like you have the right format but maybe something not quite right if it isn't working. Also though there is
http://www.wowwiki.com/API_FontInstance_SetTextColor which you can use to color a full fontinstance object.
For text portions though this is what I use in my TOCs.
|cffeda55fYourTextGoesHere|r
It might be that the || in between the |c and |r in your code may be messing it up somewhat.
D'oh, beaten by Katt rofl.