You can't see me!

Well, not if I'm painted just the right shade of green and you're reading this of a standard monitor. Monitors show colour by mixing red, green and blue light sources, so-called primary colours. Mixing them in different proportions produces a huge gamut of colours, but nonetheless, there is no mixture that produces wavelengths around 500nm - or "green" to you and me. To get that, as I understand it, you need to either have a negative amount of red (impossible if you're just adding light to other light) or else start with super-saturated light sources (also impossible). A normal RGB monitor will of course show green, just not certain wavelengths of it.

It also might be true that if you balance a scared chameleon on the back of a randy squid, it will die, for one of two reasons. Squids attract partners by changing their skin colours, often quite rapidly and extremely. Chameleons change their skin colour to blend in to the background, especially when frightened by a potential predator. This takes a lot of energy, so the chameleon will rapidly exhaust itself and die. Either that, or it will drown first.