When I try to imagine a simple solid yellow circle fixed in place I can't. Immediately my mind transforms it into something else. It starts disintegrating into different parts, changing color, moving, the background changes color, it starts flashing, it turns into a tunnel or bends in 3D space, it turns into 10 different circles and shapes or all of the above. I suppose this shows that my mind is chaotic. I'd love to have control over the images in my mind but for me I'm just an observer with little influence. Even a solid colour is challenging but I can usually hold a faint solid colour with no shape for a few seconds.