We are told by sakura at the start of the series and again by pa frog in the middle of part 2, the continual use of chakra exhausts the user's stamina(psychical energy/life energy what ever word you prefer). So once the user rests their stamina returns and so too their ability to mold chakra again.
Before I go any further the pages where either Jiraiya or Minato talk about what is actually sealed are commonly misunderstood. After the forth hokage was killed by the death god he later mentions to Naruto within his subconcuss he sealed half of the fox's chakra where chakra is made up of the two halves spiritual and physical energy. Jiraiya earlier in part 2 before he died against Nagato told us in fact the spiritual energy(yang) half was sealed within naruto by Minato. The red chakra we see naruto using while in fox mode is the fox's but as to how the fox uses his chakra despite the fact he only access to his spiritual energy is what I'm theorizing.
There are key differences with regards to the fox while he is in Naruto's body that can't be forgotten. In my opinion the way Naruto used(pre-so6p seal) the fox and vice versa is quite complicated and underrated by most. Naruto is a very unique host especially if you compare him to killer bee.
Chakra is created by mixing spiritual energy and physical energy together, how well you do this determines how good your chakra control is.
As we all know before the Fox was sealed within Naruto he had his full chakra (spiritual(yang) and physical(yin) energies). Minato sealed away the fox's physical(yin) energy within the death god and the spiritual(yang) energy within Naruto for him to use. Other than the fact Jiraiya explicitly said this, this is backed up further by the fact that Naruto(pre-so6P sealing) could only access the fox's spiritual energy via highly intensified emotion such as anger. The same way how Sasuke's evolution of his sharingan is driven by intensified emotion and we all know now the sharingan represents the spiritual energies of the so6p.
We also know Minato setup the seal in such a way to leak small amounts of the fox's spiritual energy over time. We were told Minato's seal was put in place to make sure Naruto didn't get taken over by the fox in his younger days until he was ready to take on the fox mentally and forgive himself for feeling dark emotions subconsciously about his village.
But lets look at this from the fox's perspective, he is character in the series and wants to be free.
Originally we all thought the red aura Naruto had while in fox mode was the fox's chakra as absolute. But spiritual energy on its own can't create chakra. It became the red chakra we came to love when the fox molded his spiritual energy with Naruto's physical energy. Which makes total sense as Naruto was barely in control in most of his fox-tail modes and this is further backed up how Naruto did a one handed rasengan while in fox mode against Sasuke. Now that the fox can mold chakra and can replenish his chakra with the help of Naruto's body he has an opportunity to reign havoc in the world once more. Jiraiya's statement about Naruto in 4-tail mode was "he is like a miniature kyuubi" further show us what the fox's plan was all along. And just because Naruto has resealed him now this doesn't mean his role in the story is over. After Naruto stole a large portion of the fox's spiritual energy the fox was still able to create the largest ball of energy we have ever since in the series to date.
I think kishi has already shown us Naruto's physical energy and the fox's spirtual energy merging for a chakra blast in four tail mode where the fox is firmly in control of Naruto's body.
I believe the blue balls are Naruto's physical energy and the red balls are the fox's spiritual energy, originally I though the red balls was Naruto's blood. But then it hit me, it can't be blood because Naruto's blood burned when it came into any contact with the fox's latter transformations.