On a green screen (where all colours are shades of green), the colours (in BASIC colours), are in order of increasing intensity. Black is very dark, and white is bright green, and colour 13 is a medium green. (Thanks to [[Mark Rison|Mark Rison]] for this information)
The luminance (Y) is not exactly correlated to the actual luminance of color colour images broadcast in RGB. We have other values.
Amstrad preferred to propose a completely different image system, not comparable to a conversion to monochrome, which would have limited the number of brightness levels to 21 (for example, colors colours 9 and 6 would have had the same luminance).
They opted for a table of 27 linear brightness steps. They assigned values of 1 (1kΩ) for blue, 3 (3.3kΩ) for red, and 9 (10kΩ) for green.