During the battle between Naruto and Sasuke in the original series, Naruto got one tails cloak from Kurama. That means he could use Kurama's red chakra. When Naruto uses his regular chakra of blue colour, his rasengan looks blue. But it didn't turn red when he had one tails cloak. Why?
While reviewing the fight scene, I'd like to point out something that I noticed.
Towards the beginning of Naruto and Sasuke's battle, Naruto's Rasengan was its normal blue color, as shown here. This occurrence was prior to Naruto gaining the 9 Tails Cloak.
Towards the end of the battle, while Naruto's Rasengan was forming, I noticed that this ones color was actually more of a purple color than the normal blue (albeit not red either). This is shown here
Purple is a combination of the colors Blue and Red. So with that information, we can deduce that the chakra used to perform Naruto's final Rasengan was a mixture of Naruto's blue chakra and Kurama's red chakra.
i think he formed the rasengan before wards he was in the 1 tail form than used it in the end the last rasengan was purple which looks like a tailed beast bomb in the form of a rasengan which naruto formed when he was trying to make a tailed beast bomb when he was in the chakra cloak of 9 tails so there is a possibility that the blue rasengan was formed before the 1 tail form was activated but a tailed beast bomb like man thats so good