spoilers for the first episode of the Bad Batch
A while back, I remember seeing a text post where someone was trying to make the argument that Rex was a modified clone like the Bad Batch. Some of their reasoning was spotty - the fact that Rex makes different military decisions and has different tactics can more be attributed to his being under Anakin than modification. But one detail in that stuck out to me - the fact that Rex is blond. Jango Fett isn't blond, none of the other clones are, and Rex's hair seems too short for him to dye it.
With regards to Omega - yes, she is a clone, but she's explicitly said to be a modified clone, so her being blond doesn't really make sense when talking about Rex. And the Kaminoans explicitly said that they had five modified clones, so Rex isn't one - so far. Honestly, it's still possible that they'll reveal Rex to also be modified later in the show.
But is there a reason, in-universe or out, for why Rex is the only blond clone?