How do you manipulate Textures from code?

I am wanting to draw a texture to the HUD, the texture I want to use is already in the game, I am managed to get it displaying fine. But I would like to change the black background to transparent and only leave the red channel. So I would like to do something like, new red = red, new green = 0, new blue = 0, new alpha = 255 - red.





Does anyone know how I can manipulate textures in this way?