I found a way to select a colour but it is a pain and can take multiple attempts. I had to hold up on the d-pad and down on the LS at the same time, to get the cursor to move down.
If you are after just the moonpool without having to find and scan the base, the blueprints are around, after a GRAND tour along the REEF with a bit of SEA TREADing along a PATH. As for the gun I've been to the area where it should and like you said…