Pitaya Red Velvet Cake became brown
Note that carrot cake isn't the bright orange of raw or even boiled carrots either.  This is especially true of the crust and from your description of partway through cooking I suspect that's the colour you're looking at.  If the inside is much better, there's still hope.

Many colourful compounds in plants aren't stable under heat, but even ignoring that, sugar and flour both brown on the outside of cakes. Both caramelisation and the Maillard reaction (the former is probably more important in cake) take place at typical oven temperatures, so you may have some success with a cooler oven.  This is a moist recipe so it may not turn out too dry, but you may want to cover it for the end of the cooking time.  A cheating approach is to cut the crusts off, then ice all over.

I've had a play with DIY plant extract colouring and the crust colour is clear (I didn't make enough extract, and was going for orange) You may want to look at anthocyanins, in fact given your comments about acid you may already have done so