Though not on the cyan/magenta checkerboard - I suspect that might also have been a solid colour on the original developers' hardware (probably a blue).
No comb filter will turn this into a solid blue. Trixter's capture card also seems to be chroma-only, in other words, luma is recovered by subtracting the notch-filtered chroma rather than the comb-filtered chroma from the composite signal. That's quite common, as non-adaptive chroma-only comb filter artifacts manifest themselves in desaturation of edges, while non-adaptive luma+chroma comb filter artifacts themselves as unsightly hanging dots, as seen in the third picture below. Here's the Boxing's title screen with all four filter types:
notch filter only:
simple chroma-only comb filter:
simple chroma+luma comb filter:
adaptive chroma+luma comb filter:
Screenshots depict the emulation of a new-type CGA with a hue offset of +15 (to make the sky less greenish). Obviously, this adaptive comb filter of mine could still be improved, as the two rows below "by Dave and Barry Murry" should be filtered as well. Right now, it errs a little on the conservative side, because filtering too much would look extremly unsightly.