The chromaticity diagram is basically a straight line between 640nm (red) and 545nm (green), so anything in between (including pure yellow around 570nm) can be reproduced with a linear combination of red and green.
RGBY televisions do exist, but their goal is to boost brightness in the yellow region, not color gamut.
RGBY televisions do exist, but their goal is to boost brightness in the yellow region, not color gamut.