Facebook is blue because Mark Zuckerberg is red-green colour blind.

The laws of physics dictate that if you stood inside a Harry Potter invisibility cloak, you wouldn't be able to see out. In real life it is not possible to be invisible using an invisibility cloak. It is a simple law of physics that interactions are two-way streets, so if you are invisible because nothing interacts with you, then you wouldn't be able to see- your retina would not intercept light. The particles that make us up do interact with electromagnetic radiation—which means that while we can camouflage objects, we cannot make them transparent to all forms of radiation. We might hide them in the microwave, or infrared, or even the visible part of the spectrum by exploiting the wave nature of light, either causing the waves to bend around objects or by causing waves that scatter off the object to interfere with each other, effectively cancelling each other out. But since this technological wizardry is usually tailored to the wavelength in question, then waves of vastly different wavelengths, like X-rays, for example, might still scatter off of them.

The spots and colours you see when you rub your eyes are called "Phosphenes". They are a phenomenon characterised by seeing light without light actually entering the eye. The rubbing mechanically stimulates the cells of the retina causing this effect. Sometimes, these phosphenes can still be seen for moments after you open your eyes.