Color bloom of my vitex tree( Chaste tree)

Question

I have a mature Chaste tree and its’s bloom is more cream with just a hint of lavender. It has full sun on west side of house but it is in a bed of Knock Out roses. Would not think that would matter. What can I do to make the blooms purple ? I have two others and their blooms are purple

Answer 1

The older strains had small spikes of flowers in pale lilac, mauve, off-white or light pink, but modern, much improved varieties such as 'Montrose Purple', 'LeCompte', or the pink 'Salinas Pink' have more intense colors.
https://www.gardeningknowhow.com/ornamental/trees/chaste-tree/chaste-tree-cultivation.htm