Do you have any idea why many orchids seem to have reddish/yellow/orange petals with brown speckles? Is that some sort of convergent evolution? On the request side, do you have a nice photo of Dendrobium amethystoglossum?
I am not sure what animals pollinate these orchids with the color combination you describe. However, if they’re all pollinated by the same animal (my guess is that they could be pollinated by bees), then convergent evolution is plausible. They could be exploiting a sensory bias of the animal to lure them to the flower and get them to pick up/deposit the pollinia. And your request is on its way!