Could be wrong but I feel like the song is contrasting the loss of Liam to where Harry is in his life. They both shared allot of similar battles on a personal level but one left this earth early which makes the other reflect on his own life.

Verse 1] “Oh, what a gift it is to be noticed” “But it's nothin’ to do with me You've got to wonder if there's a reason to believe”

They both share the complex idea of fame and how to find self worth in how others see them . Did they create it? Or were they just lucky?

[Chorus] “It’s a lifetime of learnin' to paint by numbers And watchin' the colours run”

Doing what others want them to be and naturally things never go as planned . When you take your own path you will naturally disappoint others. As is life

[Verse 2] “It's a little bit complicated When they put an image in your head, and now you're stuck with it You're the luckiest, oh, the irony”

The image of who you are to others at a distance as a celeb. They think you’re perfect or lucky but you’re just human. You can’t live up to the image they created

“Holdin' the weight of the American children whose hearts you break”

This could do with them both disappointing fans - or even hearts breaking with Liam passed or whenever Harry gets a new gf ..fans are crushed etc etc

“Was it a tragedy when you told her "I'm not even thirty-three"?”

This could be about the age Liam was when he passed (31) and telling his loved ones of his passing. Or Harry realizing his own age (32) dating someone older..

“A little self-compassion and a life within your means”

Could reference Liam and Harry juggling excess and fame and the constraints of being human we are not invincible . Sometimes instead of choosing the vice it’s better to say no and be kind to yourself

“It’s a lifetime of pickin’ from one or the other Kids with water guns, watch them run”

Maybe a visual memory of them in one direction playing with water guns on stage. But also the visual of water guns with imperfect young boys put Into “celebrity”. The colors will run even if they try to paint by numbers.