Ideally they will have three colors and there will not be a consistent "pattern". We often see the BF pattern in a multi fish. As long as the pattern only shows in two of the three fins we are good. If you get say a red band in all three fins.. but no clean separation in colors so figure the multi is where to put them they will be moved to the BF class as with three fins it is officially a BF with a bad pattern. Same will happen if you have a thin white or other color band on the edges. Two fins ok.. but add the third and you are back as a non competitive BF.
And red wash is also not acceptable. A blue fish with a lot of red is not a multi but a bad blue.