=== Green Screen Colours ===
On a green screen, where all colours are shades of unsaturated green, the firmware colours (in BASIC colours) are in order of increasing intensity. Black is darkest green, bright white is brightest green, and firmware colour 13 is a medium green.
The luminance (Y) is not exactly correlated to the actual luminance of 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, 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.