Why is it 'black & white' ?

Why is it 'black & white' ?

My 2nd screen only has an HDMI input, so in order to transmit the GameCube video output to this screen, i have 2 solutions :
> convert the 3RCA output of the GameCube into HDMI, which introduces lags between controller inputs & video display (not optimal for me)
> use the 3RCA video output and input it in the 5RCA luminance input of my AVerMedia capture card, reducing the lags to the minimum, by only keeping the grey shades of the GameCube signal

I chose the 2nd solution for better speedrunning capacities.