Your eyes move. Your right eye sees a green duck, and your left eye sees a violet duck. Can you make them merge into a green/violet duck?
When our eyes are presented with two slightly disparate images, we are usually able to consolidate them into one single image, where the discrepancy between the two images is experienced as depth. Since the eyes are set slightly apart, that is usually how we perceive the world. Either eye usually has a slightly different picture of the world, except when we look into the far distance.
The brain merges these pictures (the resulting product is useful, in the form of stereo vision). When the eyes perceive two images that differ too much from each other, the brain is unable to merge them. Instead, the perception may alternate between the two images. This is called binocular rivalry.
The image with the greatest contrasts will be visible the longest. A moving image, moreover, will dominate strongly. When it comes to colours, it is not as obvious which colour will dominate. Which duck dominates your vision? Or are you able to combine them into a green/violet duck?
Another fascinating question is if you are able to alternate between the ducks at will. Can your frontal lobes govern what takes place early on in the visual process?