I think there is a ???90degree (maybe 120degree) red and green on the front of the tab.
Depending how they mount (as whisper says - you need the boat there to look) they might mount on the side of the frames. Then I'd want to move the tab to be horizontal and stick an all round white on it.
I wouldn't put the white directly on top of the coloured as it will bleach the colour out to the distant viewer...
Never had to feed a wire down an a-frame but assuming the tube is hollow inside it *should* be straighforward for the side ones. Drill the hole in the tube at light level. Drill the exit hole at the base and feed some U shaped thin stiff wire into the bottom hole. Push enough in that the wire will go round the inner circumfrance of the tube. Mark a length of fishing line with the distance between the two holes. Put the fishing line with a weight on it into the hole and let it run past the mark. Assuming gravity is working (!!) your weight is below the wire U - pull the both sides of the U and you should get the fishing line... use the line to feed the wire.
Top wire will be harder, ideally use the green red wire already there to do the job. Stiff wire needed to push along the horizontal (or maybe the weight with a good tilt on the boat to the appropriate side). Then use the hole for the side light as an access point to do the vertical.
Wire size - depends on what light you fit... if its LED it could be a lot thinner... Several factors will affect your choice of wire thickness:
Current Draw. Most nav lights are 10W bulbs, which allowing for a low battery would draw about 1amp. For safety you might assume 1.5amps.
Cable length. You get voltage drop over wires which is more pronounced at lower voltages (i.e. 12V systems). You need to look at the cable run. Where is the battery, the switch and the ducting route. I take it thats a 6m RIB? So 5m from console to the frame, alow a 1m from battery to console? and then 2m up the frame? Thats 16 there and back ~ 55ft.
Is each bulb running its own wire to the battery and back or will you run say to the A Frame and then split the port & startboard (you never need to switch red on without green) so in effect you are running 20W? Or run all 3 (red, white & green) back through the one ground wire - 30W. That is still only 3amps.
Size of fuse? I'll guess the smallest fuse you have is about 5amps. So the wire needs to run 5amps otherwise the wire melts before the fuse blows if you have a fault!
So if we assume a current draw of 1.5amps/bulb and a cable run of 55ft, ( Stealth 316 - Wire Resistance and Voltage Drop Calculator
) Most people say you should aim for <2-3% voltage loss (= 0.36V), not sure that for incandescant bulbs they are that fussy but you are probably looking at a 16 AWG (multistrand) wire for a single bulb or 12 AWG for all three using the one wire. A 12 AWG wire should be OK with a 5amp load on it so a 5amp fuse would be OK, and I think most 16G wire would be OK at 5amp too but worth checking when buying that its OK or use a smaller fuse.