Texture Map Color Changing

I’m back and have another simple question for someone who knows way more than me in OpenGl. I have a texture map. For simplicity lets say it is of the letter ‘M’. The texture includes a halo around the character M within the texture ‘M’. I generate the texture with a tool so there are some hidden things going on that I’m not aware of. Anyway…the text is white and the halo is black. How can I easily swap the color of the halo and the character ‘M’ without regenerating my texture. It seems I could use something like:

where haloColor is an array of 4.

However this does not work. Can anyone help?