"my blue, and your orange... I guess something will remember us." --River Song, Archipelago (The Ninth Doctor Adventures: Star-Crossed)
they are intertwined, always.
she is blue, like the TARDIS - his TARDIS, her second mother. blue like the diary modelled after it.
he is orange, like the life she breathed back into him, lifetimes sealed in a single kiss. (clock that blue light flare on river too-!!)
blue and orange. orange and blue. the colours where they come to life.
even his TARDIS interior is blue and orange: the echo of a timeline that still lingers in soft whispers.
from an artistic perspective, these colours were probably only used together so often because they're complementary, which can add depth and distinction to a shot. but still, it feels meaningful: they're complementary. just like river and the doctor. so different, and yet they look and work perfectly together.
now, if you'll allow me to stretch the point a little further, let's go back to lake silencio, at the junction where sand meets water:
(i'm calling this a stretch because i'm well aware the sand is more yellow than orange, lol. but those colours are analogous so... i figured the idea was worth keeping in)
she emerges from the water, so cold, so blue.
he descends from the sands, golden and warm.
they meet where land meets water, orange yellow meeting blue. he is the sun and she his sky, and at 5:02pm, the universe folds itself around them, curling in on the point where these colours converge.