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1. The octopus does not sense "brightness" at all. It's sensing invisible/unperceived heat radiation from the Sun. Humans and other beings "only perceive" the invisible heat radiation as brightness. In whatever you were reading it's not the author's objective to explain that wheather he understands it or not. When people talk about the Sun's radiation they tend to put it in common terms that most of us understand which is "brightness". The Sun emits a radiation wave in the form of heat. Rocks retain heat, plants capture the heat energy (photosynthesis), animals/humans eat the plants then the "heat energy" is transferred into the animal. Brightness and color is not a property of that heat energy. Beings consume the heat energy from the Sun via their bodies (mouth) "by eating" and absorbing the heat energy via the skin. It's similar to a television getting it's power/energy from a wall outlet. Some of that energy is eventually converted back into visual perceptions by our minds.

Our bodies/minds use somewhere in the ballpark of 30% of our calorie intake and around 30% of our "brain" to produce visual images, color and brightness. If color and brightness were already present in the natural world/universe it would not consume so much of our body/mind systems energy to produce color and brightness perceptions. Beings have to feed their mind with large amounts of energy to reproduce this information into visual perception in the same way a television gets power from a wall outlet.

After animals and plants die that energy in our bodies is dispersed back into the world in a non-cohesive manner since there is nothing to hold the energy together. This explains why there is no such thing as a "soul". After we die there is no vessel to hold the energy in our bodies together. The energy that makes up our "self" or our "consciousness" would simply disperse back into the world in a non-cohesive manner. Our "self" or " our consciousness" cannot exist without our mind/body as a vessel to contain it and keep the energy cohesive. Ok, I'm getting a bit off the primary subject a bit.

The Sun's waves do not contain color or brightness as a property. Sun radiation contains invisible information "wavelengths" that our eyes and mind convert into brightness and color. Our bodies intake the energy required to make our brain/consciousness function. Our bodies are the power plant. That energy originally comes for the Suns radiation. Without our body/power plant to feed us energy we can not exist as a conscious being.

Cameras also capture invisible radiation, not color or brightness. A television does not actually output color or brightness. The TV and camera "like a prism" does not output color or brightness. They output invisible information in the form of radiation wavelengths. Our eyes and mind convert that invisible radiation into color images.

2. When you close your eyes some radiation information still makes it through to your eyelids if you are around a sufficient radiation source. People born with a detached optic nerve or detached retina do not see anything. They see what your nose sees, which is nothing. No black, no brightness, no white, no static no color. Without eyes and a mind to perceive the Sun's radiation the world and universe does not "look like" anything. Without a living being present the Sun's radiation is simply "unperceived". In the unperceived state there is no color or brightness. Visual perception of color and brightness is unique to living beings.

Yes, blind people that have perceived color before they were blind can store that converted-information/perception in their mind and recall it after they are completely blind. I also believe you are correct in that people who are blind from birth do not dream in images since they were never able to convert this invisible/unperceived information into a visual image. They have no concept of what a visual image is. They have no stored images in their mind to start with. I believe people who are blind from birth dream in sounds, smells, tastes and feelings. I'm not sure what you're point is there. This 100% agrees with what I'm stating.

Here's another way to look at it. Look at your television system. Do you see any color or brightness coming in thru the satellite dish, antenna, cable or power cord. No. There is no brightness or color going into your television system. Look at a satellite dish, notice there is no color or brightness streaming into it. Your television system is receiving information that is invisible and processing it further into more "invisible/unperceived information" that your eyes and mind can eventually interpret as color and brightness. A television works somewhat like the mind but the television is only capable of processing invisible/unperceived information and converting it to another form of invisible/unperceived radiation. The information your dish/cable/antenna is receiving can be digital information. Your TV system converts the digital information back into analog invisible/unperceived radation wavelengths that come off your TV screen. Then our eyes and mind can further process that information into color and brightness. A television does not output color or brightness. Our minds only perceive it that way.

Again, your cell phone signal does not contain "sound" as a property. Just like the Sun's radiation does not contain color or brightness as a property. The cell phone converts the invisible/unperceived information into a pressure wave that propagates via the air and our ears/mind convert that pressure wave into sound. Sound only exists as a pressure wave before it's perceived by a mind and converted to sound. A cell phone single propagating thru the air does not contain sound just like the Sun's radiation does not contain color or brightness.

The question "if a tree falls in the forest does it make a sound" is a bit ambiguous and confusing. Partly because the word sound in the dictionary is defined as a "pressure wave". To me the pressure wave is one thing that is outside the mind and sound is another that only exists within the mind. However since the word "sound" is defined as a "pressure wave" in the dictionary the statement can be interpreted as "sound being something that exists outside the mind". The answer simply depends on how you define the word "sound". Either way "a pressure wave exists in the world outside the mind" and "what we experience as sound" only exists within the mind.

I am writing this "on the fly". If see something that is incorrect or unusually confusing I'll try to correct it the next time.