Here is my answer:

var ls = List('R','G','B')
var res=scala.collection.mutable.ListBuffer[String]()
for {i<-ls; j<-ls; k<-ls}
{

  if (i!=j && j!=k && k!=i) {
    var x=s"$i$j$k" // string concat
    res+=x
  }
}
println(res.toList)