Color output into Value socket
mmin 6:00 you say that when you take a yellow output and plug it into a grey input socket, it takes the Value of the Color. This is not correct: it takes the Luminance of the Color. The same would be if you insert a RGB to BW in between (that node also calculates the Luminance). So it is a rather unintuitve result and imho should be avoided (the other way around is perfectly fine of course).
Blender calculates the Luminance according the ITU-R standard: 0.2126R + 0.7152G + 0.0722B.
(Actual luminance is subjective...)