MagicMix The well known effects of colours on the psyche aside which one might seek to benefit from, no, no one needs coloured walls. I completely agree with you on that.
And it is also not a reflection on anyone's parenting skills whether you paint or wallpaper and in what colours or patterns.
However, when our oldest was born, nursery - shall we say trends? - were pastels. It looked like everything had been bleached, all colour had leeched from the environment newborns were put into.
At the time, studies were published showing that newborns showed very little reaction to pastels and a lot to bright colours. We didn't need brainscans for that though, did we? It's no coincidence that bright colours have been used for children's things for so long.
Our boys have always loved bright colours, complaining from a very young age that they didn't want clothes in the prevailing "boys' colours" of dark blue, dark green, brown and black.
So, yes, walls are largely irrelevant, you can bring bright colours into their environment in other ways. But in my experience, an astonishing number of parents colour coordinate the nursery far beyond just walls, even when pastel colours can barely be differentiated by newborns. And when I talk to them about a newborn's capacity to perceive colour, they're often surprised as they just didn't know.