Use the white PVC ...

Good Day ...

gray is probably fine. Actually gray is sunlight resistant and white isn't. Keeping in mind - Gray is not drinking water safe.

schd 80 is thick, so 3/4 may be closer to the inside equivalent flow characteristics of 1/2

Fittings will not match up, nor is electrical conduit pressure rated for liquid;

I used some schedule 40 white pipe plus some schedule ?? gray for running my electrical wires and they matched up OK but I changed them later after someone told me the reason for the colors. I do not know what schedule the gray pipe was but it came from Home Depot. The gray did have a pipe that I really wanted to use but did not do it. Instead of the white 90 degree turn, I liked the gray radiused turn better. Figured they might carry water more efficiently.

I wanted to be able to identify the pipe carrying my wires if I ever have to do that. They all are in the same trenches.