How to prevent brillant pink raw bread dough from cooking up to yellow?
It appears that the pH of your bread changed during baking. Beets are red because of their anthocyanins. According to Wikipedia:

Anthocyanins can be used as pH indicators because their color changes with pH; they are pink in acidic solutions (pH &lt; 7), purple in neutral solutions (pH ~ 7), greenish-yellow in alkaline solutions (pH > 7), and colourless in very alkaline solutions, where the pigment is completely reduced.[5]

You would have to post your recipe or analyze it yourself to determine what is alkalizing your dough, perhaps baking soda? Changing your recipe to make it more acidic may change the texture
of your bread. You could look for another recipe that uses acidic ingredients, such as butter milk that might off set the color change. The pink looks very pretty though.