Note that “luma” (Y’, to avoid confusion please don’t call this Y) is based on a gamma-adjusted RGB space, and is therefore is not accurate for saturated colors

Note that “luma” (Y’, to avoid confusion please don’t call this Y) is based on a gamma-adjusted RGB space, and is therefore is not accurate for saturated colors. To get a reasonably accurate correlate of lightness perception, you need to take your linear combination in a linear RGB space to get luminance (Y), and then afterward apply a non-linear correction to find lightness.