It would be less to do to Red, but it would be more difficult technically because Red uses ASM for events while Gold has an internal scripting system. But the real reason for doing it on Red is because the entire point of Secrets and Rumours from the very beginning was for it to be like Red Version, but this time to have the Pokegods, Mist Stone, Brick Town, and all that stuff that we made up back in the late 90s and talked about.
And honestly, the lack of color is not a problem for me. I play Red on my android and the black and white doesn't bother me, it adds to the nostalgia (which is of course the only reason Secrets and Rumours exists haha).