I've read that the human eye can only really decipher around 10 million different colours, so what's the point in technology that uses 48-bit colour, which allows for $2.81\times 10^{14}$ colours. Is this not like giant, super inefficient overkill?
What's the point of 48 bit colour?
graphics;hci
Some reasons for using more bits per color channel:Tetrachromats. There are rare humans with four types of cone receptors in their retinas instead of the usual three.  These people can distinguish an order of magnitude more color shades than the rest of us.Color gamut.  Your question assumes that the 16 million colors available in 8-bit per channel RGB exactly covers the colors that normal humans can see.  But if that were so then we would only need one RGB color space.  We have more..  More bits per channel allows a wider gamut of colors to be represented without posterization, because the increased bit depth means smaller gaps between the individual shades.Image manipulation.  Every image manipulation, from switching color models for printing, to sharpening, to resizing, to tone curve adjustment, loses information.  The more information you start with the more you have at the end of post-processing.  The more information you start with the easier it is to avoid the introduction of visible artifacts like posterization.