That's true. In my case, no speakers are more than 5' apart, as I centered them within the room [1/3 in on each side]. I don't have rooms that are 15' wide, so that's the max. What I did was to run the sleeve to the center, so that the exposed run was only 2.5' wide max.
You used the words "The builder might suspect especially the colors black and red" - what does that mean?
The wires are black, green, white, red. I used the follwoing standard not just for this wiring, but also all the wiring in my house - red & green are positive, black & white are negative. Hence red/black go to one speaker, white/green go to the other.
As I type this, I realize that it would have been even nicer had I standardized on red/black for right, green/white for left.