The company colour-codes its boats!
Yellow flags stop only at these ten piers:
E and W represent the side of the river the pier is onN30: Nonthaburi (Pibul 3) (E)
N24: RAMA 7 (W)
N22: Bang Po (E)
N15: Thewet (E)
N12: Phra Pin Klao Bridge (W)
N10: Wang Lang (Siriraj) (W)
N5: Rachawongse (E)
N3: Si Phraya (E)
Central Pier (E)
S4: Ratburana (Big C) (W)
Now to find those red-roofed buildings? (And that's a farang couple in the background, we can't be too far out of the city.)