I think the Skwawkbox lot are being a tad disingenuous here.
This is where they got it from:
https://www.theguardian.com/politics/2021/aug/21/keir-starmer-ill-paint-a-picture-of-my-vision-in-primary-coloursStarmer did say it but it doesn't sound like an ongoing slogan just part of a sentence. I may be wrong, of course, but I can't find a reference to it as a slogan anywhere else.
Plus the Skwawkbox have had it in for Starmer since the sainted JC left.
I'm not defending it because I think it sounds clunky and ugly and far too American for my liking, but there is a bit of mischief making going on.