How can I delay a digital signal
You can use a logic delay line. This Maxim part ought to do it: https://www.maximintegrated.com/en/products/analog/clock-generation-distribution/DS1100.html

Another idea. If you have a common clock that makes the sync timebase (say, 4*Fsc), and it's common with your GPIO timebase that makes your RGB (it would have to be, right?), could you delay the mux signal using flip-flops?