Purple Sweet Potato Bread

I've tried this twice and failed.  Failed to get a good purple color like my awesome muffins and scones.  Something must be happening to the mix during the baking process because it starts out like this:

Then after baking it looks like this:

Kinda green in the middle, slightly purple on top.  I won't share the recipe, it wasn't outstanding.  Next year I'll try again as I've nearly run out of Purple sweet potatoes and the kids won't stop asking for more purple scones.