Any biological process is like watching a city being built. There are
things happening at all scales, from bolt tightening to the movement of large steel girders, that ultimately result in a structure that is both incredibly intricate and dynamic. Colour vision is no exception to this idea.
We will start at the “bolt tightening” scale of a single molecule and ultimately work our way out to the electric currents that actually run between cells. What you see below is a single molecule of what is, essentially, vitamin A (retinol). A photon of light is absorbed by the molecule and a key double bond flips from the ‘cis’ conformation to the more stable ‘trans’. This ‘switch’ is the first real event in a cascade that leads to seeing a color.
There are some lies here (e.g. the double bound doesn’t rewind to the cis conformation by itself). We’ll keep taking steps back in time and space to see the bigger picture of vision (limited to how fast I can learn enough Synfig to animate it). This cascade amplifies this tiny tiny molecular event so effectively that you can see a single photon under the right (very very dark) conditions. Think about that, your brain can detect the movement of a single chemical bond!