I got sent this picture by my sister friend who is getting married. However she only wants buttercream. I can't tell if this is ribbon, or if it's painted on. I'm thinking it's ribbon but I'm not sure. If anyone has seen this, or has done something similar to this on buttercream, please let me know how you did this!

Thanks in advance!

It looks like strips of fondant to me. 

You can do that in ribbon or fondant.  Whichever you prefer.