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That was how the predecessor 8088 MPH worked. Area 5150 however is aimed at a TTL RGBI monitor, and as such, does not use NTSC composite artifacting. It can only display 16 colours at once. So in this demo, you get 640x200 in 16 colours, where additional colours are implied by using dither patterns. (Technically 8088 MPH did more or less the same, except these 'dither patterns' would result in NTSC artifact colours).

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Thanks, I should have written "more than 16 colors", forgot that CGA has a 16 color mode without tricks.

Dither patterns/"artifact colors" are exactly what I meant, they achieve color by modulating the luminance signal (which is of course the same as "normal" color, but the trick is to let the software influence only the luminance and getting "unintended" colors instead of directly telling the CGA adapter to modulate the color on, and over a separate circuit probably, like what normal 4/16 color mode does).

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