I don't think I really realized it, because it happened gradually season by season. But during season 7, I watched an old episode and then it was really obvious. Tyrion and Jaime were "Lannister blonde" at the beginning, but ended up with much darker hair. Tyrion even got curlier hair with each season.
Tyrion season 1:
Tyrion season 8:
Jaime season 1:
Jaime season 8:
Tyrion season 1:
Tyrion season 8:
Jaime season 1:
Jaime season 8: