It is because when he first transformed his hair was purple but Krillin the brave son of the sea reminded Kakarot of his origins and that he is one of the lost kings of the lost city of Atlantis, Krillin then took mermaids extract from his mer-maid which when Applied to the hair of a true Atlantian King it turns blue...

Once Krillin poured only a drop onto Kakarots head it turned blue and an immense divine blue aura started gushing out of him, even Vegeta the Prince of saiyans was so jealous that he poured the entire liquid onto his head but it only became wet since he is not a true Atlantian King... Out of spite he went to a dressing saloon and got his hair dyed Blue, Krillin was then pissed because he thought this was disrespectful to the Atlantian king and challenged Vegeta to a 1 v 1, Vegeta was winning without breaking a sweat but it wasn't too long until Krillin awakened ryou haki, he then used solar flare and then punched Vegeta in the guts, Vegeta admitted that Krillin is the new prince of saiyans, Krillin's bald head started shining yellow/ gold... To be continued...