Thanks for your answers.
@nmalinoski – I’m inclined to say it’s the cables too, they are cheap and as you said i checked for audio hum and indeed there’s a massive audio hum on white screens (or any screen other than black really)
@SegaFanboy – I do use the default power supply for OSSC as i ordered it with the device. For my consoles i do use the default one too except for the mega drive, but the problem is showing with all the consoles, not just the mega drive (it’s just worse with that one)
I bought all the SCART cables for all the consoles in the same store and they were all equally cheap.
Edit: I decided to open a cable, here are the results!