It could be a pumpkin that is transitioning from green to orange. But since they're volunteers you have no idea about the parentage. You can't know who the pollen plant was or whether either parent was open pollinated. So you're going to get what you're going to get. I think you can reliably hope for a nice jack-o-lantern.

Yeah that looks like a pumpkin that's just starting to ripen.

nice job!

Yep, that's what pumpkins look like while they're ripening. In my experience volunteers from store-bought pumpkins usually do end up as normal orange pumpkins.