I agree with Jackster that if you are going to have a single body color, rusty brown is the most important. I will also agree that if the spinner fall is during dusk or at night, the fish see a silhouette, and body color is not all that important.
However, I will disagree that in some situations that I fish, trout are not selective to body color.
Here is an example from this summer in Montana on the Madison River. I was fishing the Colonel's Pool with Gary Borger. The Colonel's Pool always contains fish and they are feeding on a smorgasbord of insects - both caddis and mayflies in various stages.
Here is the what the Colonel's pool looks like from downstream. The current flow enters the pool along its left edge and creates both upwelling current and a counterclockwise eddy.
Here's Gary fishing it from the upstream position to fish feeding on caddis emergers. The main downstream current is to Gary's right but there is also and eddy current that rotates upstream to his left.
I was close to Gary's position and fishing to the eddy current on his left side. There were fish feeding on Epeorus (pink lady) spinners. The Epeorus spinners are not rust or sulphur in color. They are a tannish pink. This was during the day and the fish can see the color of the spinner just like they can see the color of an adult PMD during the day. I would not fish a rust colored pattern during a PMD hatch and the rusty spinner does not work well when fish are feeding on pink lady spinners during the day. This is especially true when they have forever to examine the fly as in this situation.
Here's the Epeorus spinner pattern from BRF. If you look at the color of the dubbing at about 2:30 in the video, it matches the color of the spinner in the photo below the video.
Here's a photo of a pink lady spinner from troutnut
I think spinner color is one of those common instances in fly fishing that is situationally dependent. The answer will vary with the person answering the question. The correct answer for you depends on what situation you find yourself in.