Did you mean to respond to another comment in this thread where they were talking about YUV? Your comment does not make much sense to me here but would make more sense to me there.
Luma is an approximation of perceived brightness. All the conversion formulae weigh blue substantially less than the other primaries. This supports crazygringo's assertion that "we perceive primary blue as a much darker color than primary red or green".
It makes sense to me here - GP discusses how we see perceive blue as a dark color, and parent comment corroborates that with a low luma coefficient for blue.
