Naturally a follow up question would be if there's a good source, which supports the claim about the scout ships.

This comes from Vegetius, who wrote a military training manual in the IV century AD. He never mentions Caesar, though.

This is the passage (Epitoma Rei Militaris, IV, XXXVII):

[...] Ne tamen exploratiae naves candore prodantur, colore Veneto, qui marinis est fluctibus similis, vela tinguntur et funes, cera etiam, qua ungere solent naves, inficitur. Nautaeque vel milites Venetam vestem induunt, ut non solum per noctem sed etiam per diem facilius lateant explorantes.

And this is my (pretty shoddy) translation:

To avoid giving scout vessels away for the whiteness [of their sails], sails and ropes are dyed with Venetian color, which resembles the waves of the sea; even the wax they use to caulk the ships is dyed. Sailors and soldiers wear a Venetian colored uniform, so they can more easily avoid being spotted while scouting day and night.