Composicion en After Effects con fondo blanco cambiar de color a negro en ME

You have changed the Composition Background color to white. Setting the background color does not create colored pixels; it creates a color bias for alpha channel edges when you render using pre-multiplied with alpha. Most codecs ignore the background color completely. If you want a white background, add a white solid to the timeline and put it at the bottom of the layer stack.

Please let me know the settings of the .avi format i.e. codec etc. Also, it would be great if you can share a few screenshots of AE with the timeline and composition panel in view and screenshots of AE project settings, Media encoder export settings.