i dont know where you got that idea. that is not true show any colours you want though i see through the years of showing calls in 4h that white calls do better. im in 4h and have shown a bunch of different colours.
There are no restrictions on breed or color in 4-H. Any color variety, even those not recognized by the APA, may be shown in a 4-H show.
Call ducks do wonderful in 4-H. Their small size makes them easier for smaller kids to handle. They also have those cheery dispositions.
There is no one variety of calls that will do better than others. White calls, in general, tend to be better quality than other less common colors. It is easier to find show quality white calls and so most of the white calls seen are better quality do to availability in most places. In the end it all comes down to the quality of the bird itself and not the color.
I have been showing for 7 years in 4-H and have never been turned away because of the color of my animal. I've even shown crossbreds, which are very common in 4-H.