I have never had any difficulty seeing ships painted grey.

No need to apply surface coat?! So that all recon and other air-based assets of the enemy should have no difficulty to spot the Russian warships from virtually their naked eyes?

And what makes you think a ship painted grey would be invisible from the same position?

The difference is zero and if one saves time, effort and money the real question is why does the rest of the world waste money painting their decks grey when anything that can see the red would see the grey anyway. Now if tehy were painted a blue colour that always matched the sea that the ship happened to be traversing at the time then you might have a point, but I have never seen the sea Navy ship grey and I doubt it happens very often.