How to get back to default home screen

Hi,

I have an iPhone 12 running IOS 17.4.1. Currently I've got a personal picture on my lock screen and the default blue background/gradient on my home screen. I want to change the picture on my lock screen (previously not a problem), but to leave the default blue on my home screen, but I don't see to be able to do this.

I can change the picture on my lock screen but my phone keeps asking me to customise the home screen but isn't giving me the option to go back to the default blue. The only blue option now are either a garish bright blue or blue purple.

Is there a way to only change my lock screen but not my home screen?

Failing that, how do I get back to default "old" blue on my home screen? The "old" blue that I mean is a mid blue at the top fading to a pale green/grey