How does baby hair colour change so radically?

Although.....red haired babies stay that colour all their life.

Mine is the exception then. (Doesn't surprise me at all )
She started off auburn but it grew out platinum blonde.

My mother was delighted that I was born with jet black hair, just like my dad. By the time I was 2, it had turned almost white blonde.
By my teens my teens it was mid brown and stayed that way until I started to go grey.
Both my children started life with blonde hair which darkened. They both have dark brown hair now.

My daughter has 4 children all with different coloured hair. One is strawberry blonde, one auburn, one brown and one very blonde. They were all born with the same colour hair they have now.

Both I and DS were blond as toddlers and got darker as we got older. By the time I was in my teens I had dark hair.

Now it's gone light again (but not blond!)..... DS is headed the same way, grey early, like me.

Mine all began with blond hair, one had hardly any until a year. They have all gone variously dark and now greying but the one with the lightest hair which was late coming became the darkest and was the first to have a receding hairline. Poor man.
It is curious isn't it.
People cannot believe that my own baby photos of the blonde cherub variety, are actually me.

Although.....red haired babies stay that colour all their life.

My son was blonde as a young child and his hair started to darken when he was about eleven. It is now dark brown (we won’t mention the - erm - salt and pepper!). DD had a shock of dark hair as a baby and it has stayed that way. GS is blonde like his father but his hair has started to darken a bit this last year. I think it’s just a natural process, but I hope it doesn’t go too dark.

Yes, it’s common for this to happen OP.
My DGS was blonde until he was about six, then golden-ish for a few years, then light brown for a while, and now at 16 he has almost black hair.

Surely most white British children are fair-haired when small. It usually darkens during childhood or teens. I know a middle-aged woman who still has the flaxen hair she started with.
Dark-haired children don’t change as drastically.

All of mine born virtually bald, eventually all had very fair hair which gradually darkened as they grew up, one into reddish tinge, one stayed blonde, two went into a sort of mid brown.

First six grandchildren born with dark hair which has stayed, last one bald growing into blond, and now with a little bit of his father's reddish tinge.

Two of mine were born with very dark hair, and it has never changed. The other was born ash blonde, stayed that colour throughout her early childhood, then went a few shades darker and started to have it coloured in her teens. She says she wouldn't know what colour it is now!

The DGS have all kept the hair colour they were born with, but the two that had very blonde hair at birth have gone just a shade darker. It lightens up again with a bit of sun.

One of my DGD's was born with a shock of black hair, but is now, at the age of 18 months, a beautiful blonde.

All my three DS were angelic blond babies, but all have got quite dark brown hair now - except the eldest, who is almost half grey at 38.

My GG twins are so different with the girl being flaxen-haired and blue-eyed while the boy is dark with hazel eyes. Both were blonde at birth. They were what I call double-decker twins as opposed to sharing a sac. Non identical.

Brother and I were both silver/blonde up until school then we went darker, a mid-brown I suppose.

My eldest son had bright blonde ringlets. He's now 39, has got wavy black hair

My brother and I started blonde and by the time of starting school I was mid brown and my brother was darker, though he was lighter blonde as a toddler.
My sons started blonde, one very fair ash blonde, and became light/mousy brown by the time of starting school.

My five all had lots of hair at birth and only one lost any of it M0nica.

Two were fair-to-blonde and three dark.

One dark one lightened to nearly blonde over about seven years then stayed that way.

One dark one lightened to mid brown over about 14 years.

One dark one stayed dark.

The two fair haired children both became white blond by about 2 then one darkened a little to fair and the other is now dark brown.

It makes no sense to me!

My two boys were blonde (well bald first then blonde!) but one is still blonde-ish and the other is almost black now.

Both boys have brown eyes.

I was very blonde, then I went very dark brown almost black.

Brother was born dark haired and stayed dark.

Both our daughters born blonde, one is still very blonde the other one now dark, seems very random to me.

Both DS and DGS were born with dark hair that fell out after a month or so, as baby's hair does and when it regrew it was very very blonde. DS's hair gradually got darker as he moved through his teens and by his 20s was back to the colour he was born with. DGS seems to be about to do the same.

I have always understoodthat this was the usual pattern, a babies birth hair colour being the adult colour which they lose to be replaced by a lighter colour, not necessarily blonde,

DD started dark brown, regrowth hair was mid brown and she was dark brown again by her late teens.

All my children and GC were born with dark virtually black hair they are all now varying shades of blonde.