A bathroom has, even before the kitchen, the longest lifespan (renovation interval) of the rooms in a house. That means it is good if you can look at it for a long time, and for that, it is advantageous if all the fixed installations remain rather neutral. In this respect, the dealer’s advice is meaningful. It is, of course, easy to change the color impression later when it is created through accessories. Still, the line between being able to look at it for a long time and being boring is narrow. Just go to the most expensive bathroom showroom you can find and look at the designs from fundamental perspectives. Here, a confident choice of style makes the difference.
Still, in your own home, you should build what you want to have yourself and not just what someone else’s considerations suggest. So if you like a border, then just put it up. If you like lime green, then go ahead and put it on. It’s quickly painted over if it was a mistake. Only if you are uncertain, I would get a roll of wallpaper and paint a whole strip in the desired color as a sample and hang it in the bathroom for 2-3 days. Especially with bright colors, you can easily misjudge the intensity.
A design that falls outside the "pleases-everyone frame" will polarize. But it also holds the chance that it will be exactly yours.
Best regards
Gabriele