Saw this last night, refrained from commenting because I have no actual hard answer, but since it hasn't been answered yet, I'll give it a rundown.

The aqua/turqoise colour goes back to WW2, when we see it used here and there. The theory behind it, and I can't confirm whether the Soviets believed in this or not, is that the turqoise colour is subconsciously calming to the pilots; aiding them with keeping a cool head during stressful situations.

Another part of it is that the colour supposedly facilitates the shift from "watching the cockpit stuff" to "scanning the skies" as far as your eyes are concerned. It also didn't light up much during nighttime.

There's a bunch of these theories floating around about it, most of them make a fair amount of sense. I don't know why exactly the turqoise became the 'semi-official' cockpit colour of Soviet-bloc planes, whether there was some official directive or officious consensus. However, considering how long the colour was used, we can somewhat safely conclude that it either worked as intended or was not bothersome to the pilots. After all, if the pilots had been constantly complaining about it, even the usually stubborn Soviets, who didn't place that much trust in pilot initiative in the first place (I've seen the theory of Soviet pilots having to fly more head-down floated around in conjunction with the funky cockpit colour), would've conceded at some point and stopped using that specific colour.