Greetings, how did you get on?
I expect like many, you looked up COLREGs and did the lights for PDV over 50m.
Having spent over 7 years in submarines, I know a little bit about this answer.
The complete answer is not in COLREGS but can be found in the Mariners handbook - chapter 3 - which has the definitive answer. Whilst the question asks for lights, there are a few considerations that submarines on the surface demand.
UK vessels company will fly the International Code group NE2 to denote that submarines, which may be submerged, are in the vicinity. Vessels should give a wide berth to any vessel flying this signal. If from any cause it is necessary to approach her, a good lookout must be kept for submarines whose presence may be indicated only by their periscopes or other masts showing above the water.
It must not be inferred from the above that submarines exercise only when in company with escorting vessels.
Back to Navigation lights
1. Masthead lights and sidelights of submarines are placed well forward and very low over the water in proportion to the length and tonnage of these vessels. The forward masthead light may be lower than the sidelights and the after masthead light may be well forward of the mid-point of the submarine’s length.
Sternlights are placed very low indeed, often on the rudder and may at times be partially obscured by spray and wash. They are invariably lower than the sidelights.
2. At anchor or at a buoy by night, submarines exhibit an all-round white light amidships in addition to the normal anchor lights. The after anchor light of nuclear submarines is mounted on the upper rudder which is some distance astern of the hull’s surface waterline. Care must be taken to avoid confusion with two separate vessels of less than 50 m in length.
3. The overall arrangements of submarines’ lights are therefore unusual and may well give the impression of markedly smaller and shorter vessels than they are. Their vulnerability to collision when proceeding on the surface and the fact that many submarines (all UK submarines) are nuclear powered dictates particular caution when approaching them.
4. Some submarines are fitted with a very quick-flashing yellow (amber) anti-collision light. These lights flash at between 90 and 105 flashes per minute and are fitted 1 to 2 m above or below the masthead light. They should not be confused with a similar light exhibited by hovercraft with a rate of 120 flashes, or more per minute.
5. The showing of these yellow (amber) lights is intended to indicate to an approaching vessel the need for added caution rather than to give immediate identification of the type of vessel exhibiting the light.
If you already had all all of that - BZ
On the subject of submarines, there are a few countries such as Turkey, Quatar, India, Pakistan and Indonesia that operate submarines less than 50m. They are also becoming more popular amongst those that would like to transport goods/people covertly and there have been several articles of some rich and famous opting for life on a submarine rather than a super yacht.
Have a great day.