370mm X 250mm X 300mm

[curious aardvark]

the 4th element is for white. Think about inkjet printers.
4 colours because the white is already present in the paper and you need black to adjust for shades.
Although - it would actually need 5
In life, the white is represented by - well light itself.

In 3d printing you need white and black to make shades of colours. The three primary colours themselves will mix any colour - but the lightness and darkness of shades will be fixed. The white and black are there to change the 'brightness'.

So ideally for a full colour mixing fdm setup you would have black, white, red, yellow and blue.
Quite how you would blend actual filaments together - no clue.
The best option at the moment are the inkjet solutions like the xyz davinci colour uses. A white filament that's coloured with the three primary colours and a black.

That said, looking at the mosaic, it doesn't do blending after all. Pretty much the same as the mmu2s, one colour at a time.
But I do like the way it cuts and shunts the filament into the system as it actually connects each piece of filament to the next with a proper melted joint.
I guess the advantages over the mmu2s would be mainly in speed as you just get a continuous length of filament rather than all the stopping and starting and retracting with the mmu2s.
Each length is precision cut and joined to the next in line.

So no it isn't actually extruding new filament - but it is heat joining the different lengths.