Thanks for the purchase!
Unfortunately, is like I told you before:
By using my Bitmap plugin, the colors will only change if using the Pro version. Regardless you have Visustella or any other plugin that changes text colors. Because these other plugins are made to change the color of the default RPG Maker text, which is not the Bitmap Text.
But, you will be surely able to change the colors of the text by using my Message Action(Free) plugin:
https://hakuenstudio.itch.io/eli-message-actions-for-rpg-maker
The only way that cannot work, is if it has a compatibility issue with another plugin. And with Visustella, on my side, there is no way to know or fix it because of the obfuscated code and because of their terms of use.
To confirm the compatibility problem, try on a project with only my plugins, if even there you didn't manage to change the colors, then I have to check to understand what is happening.