Change the color for opening balance or start figure in variance waterfall


when using a variance waterfall eg one category and no breakdown, how do we change the color of column1, for instance if starting with budget numbers adding drivers then ending with a forecast, i need the drivers to be red or green for up and down but the start and end balance blue, i want to avoid custom data colors as that would be manual changes each month. I heard you can add a custom theme with JSON but not sure how to still have the automatic green and red variances.

Edited question