bottom of the sea - we're talking deep sea and no I don't want black as a colour suggestion due to depth - however if I were to build a passing submarine deep below the sea and wished to show rock and plant and stuff, what coulor would be a good full all over bottom colour to pick for this diorama, look forward to your reply.

That really does depend on the ocean floor. Then that would also depend on where you are located. In certain areas the bottom would be sand from coral, rock, or lava. Those all are different in color from white to black. Of if you are near the mouth of a river the bottom could be anything from brown to black.

Of course then take into account any plants growing on the ocean floor and then things are different. But plant life mostly requires light and a submarine really wouldn’t be operating in that shallow of water.

If you are looking at a rocky bottom I would go with a grey similar to that which you see in granite.

I agree with Jim.

I would get a really dark base Probably paint it dark grey then get some (real) dark sand and sprinkle it on it spray that down with a white glu then get some real dark pieces of stone and glue them on. Plant life....none assuming you are wanting deep, not enough sunlight for most of it.