I have an interesting thing for you today.
See this photo?
That is a two part epoxy glue that I use all the time. You are supposed to squirt out a small puddle of liquid from Part A which is clear, and then squirt out the same amount of Part B (on the right), which is amber colored.
So, notice how the levels are not the same? It is because of an illusion.
I squirt out small puddles of this stuff all the time. I do my best to make sure the puddles are the same size. You can see the difference in the levels which shows I am not equal at all.
This is why. The amber colored liquid looks heavier because of the darker color, so my mind adjusts, and pours a smaller puddle of amber. The darker liquid holds more weight in my mind. When I try to pour two exact same sized puddles, I always will unconsciously pour a smaller puddle of the darker liquid.
This is a great concept for your artwork, or film composition. A darker color on one side of your photo image, or artwork can be balanced with more light color on the opposite side. A white square on one side of your painting is not going to balance out a black square. If you are trying to balance your composition, you have to take in to account that darker colors weigh more in our minds. A larger volume of lighter color will be needed to balance out the darker color.
I also notice this effect when I am mixing blue putty with light yellow putty. I always use more light yellow even though I try to mix them equally.
Go ahead, make some art, have some fun.