Orange coneflowers are hybrids, so the short answer to your question is that if they reseed in your garden, the offspring may be neither orange nor the original purple.
A longer explanation: hybrids are the result of the cross of two parents that may or may not look like the resulting offspring depending on how the genes are expressed. For instance, if the parents both have recessive genes for orange, then you will get orange offspring. But if only one parent has a recessive orange gene, then none of the offspring will be orange.
The offspring of a hybrid cross will bear seeds that may or may not germinate. A lot of hybrids are sterile. Others will germinate into plants that do not look like their hybrid parents or even like the grandparents depending on the gene expression.