PART 1: BASIC HAKI
Haki is spiritual energy given to every living creature in One Piece world. But there has been lots of mysteries shrouding it and we yet to understand it's full potential. Right after the 2 years of timeskip when Luffy is leaving Rusukaina, Luffy said he has only learned the basic. BASICS?
The Armament haki (busoshoku) is the ability to project your spirit into armor. An example of how it represents one's spirit is Vergo's purple haki as per debut. Vergo maybe mastered his armament haki. So this purple tint to his haki could've been his spirit's nature, the same to Luffy where he got his own shade. This concept may rely on the anime but just remember that the anime does have connection with Oda. So it is very likely he told them to make it in that way.
Now, Observation haki (kenbunshoku) is the ablitiy to sense spiritual energy. It is almost like the mind's eye but it goes even further than this. Mantra as per Skypeians. This was shown many times to predict an opponent's attack and also to detect its effectiveness. It is almost like mind reading. When two powerful observation users fight, it is in a way like Izanagi (Naruto). It is a battle of who can achieve better future by having the better premonition and turning the possibility into reality. This makes it just as powerful as armament haki.
Since observation haki gives you this premonition, then it should also make possible that master of it can see even farther into future. Basil Hawkins may use this to help tap into his tarot card readings. The ability to see the future and read perfectly into your opponent's moves are what make you a master of observation.
Lastly, Conquerers haki (haoshoku). This is the power to dominate others and gives you the power of a king. If we consider the fact that Luffy's ability to "attract" (Mihawk said this in Marineford war) others is an aspect of conquerers haki have loads of respect and status, that should mean that it gives the user the ability to control others in some ways. If someone disobeys you, you overpower them with your will to show dominance. Haoshoku can mix into physical attacks too, just like Shanks destroys part of a ship by just releasing his haki.
PART 2: ADVANCE HAKI
Now let's think about armament and observation haki. Armament haki is spirit armor. What would be the next thing to go with spirit armor? Spirit Weapons! (As shown by Rayleigh and Sentomaru) This may seem far-fetched at first but just think about it logically. If observation haki is like your mind's eye, and armament haki is your soul being materialized to strengthen your body, then it would make sense if you could materialize your soul into weapon using your mind's eye as guidance. It also would be boring if people only used armament haki to fight for whole series.
The reason we have yet to see this because nobody has mastered both armament and observation. Also haki has limits and can be looked at as a gauge. When observation haki and conquerer's haki combine, I have two ideas of what could happen. But first, think back to Ms Goldenweek. She had the ability to use mind control traps that influence and even enforce your actions. This wasn't haki or DF. Oda said it was pure hypnosis. This is a lot like Jango's and Brook's powers as well.
If mind control is an ability widely used by weak characters, then doesn't that mean it is possible for even stronger characters to use it with haki? Combining your ability to overpower others with your mind's eye can take control of other people's decisions. What could this be used for? Well we've seen Doffy use a power like this to turn troops against each other. This could be a result of his haki and not just his DF. Many Says it is just him controlling people like a puppet. But usually he just attaches a single string to a person, which would not be enough to control their whole body. Maybe he could channel his haki through the thread to control people.
PART 3: FINAL HAKI COLOR
Let's take what we know already. We know that Shanks, Rayleigh and maybe even Roger are haki masters who do not have a DF. So what makes them strong? Strong basic haki alone maybe can't put them up against an awakened DF user. But how can we assume they even have a final color, or what it is? Asura is not a cursed sword ability. It is Zoro's spirit, his haki (as Kaku said). Sanji might be a bit hard for some to accept but he's shown to be able to summon fire at will around his body. He even looks like the devil sometimes. Just like Asura, he has the power of a god/deity, the devil. Clearly neither is an illusion, as Kaku thought they were because they had real effects. As what Oda replied in SBS 44, "Sanji's leg is not hot. His heart is burning much hotter".
The final is, The Color of God. This does not mean that they had the actual final color of haki. It just means that their spirit was beginning to leak out, and their true forms were showing. This is what represents who they are and their will power was so strong that it began to take form.
The Gorosei have been thought to be unaging for quite some time. This is supported by the fact that they have not change in age at all in the last 20 years. So if they've been around for a long time, wouldn't that mean they've had a lots of time to train and master haki completely? Imagine how powerful they could get with 100 timeskips worth of training. One of them has even been seen with a sword. I don't think "lazy old man" sit around sharpening swords on their free time.
Also, considering the fact that Trebol said that people with kings haki are usually always leaders in the world. What better candidates are their than the leaders of the world themselves? Will the Gorosei have the color of god? well, they're called 'gods' for a reason. Much like Zoro, Sanji and eventually Luffy, they will reveal their perfected color of god. This means they have mastered every color of haki including the mixes.
Their colors of god are most likely based on the five main hindu deities; Shiva, Vishnu, Devi, Surya and Ganesha. Each of these gods uses mantra. Sound familiar? So why this is important? If the Gorosei use mantra, then it might've been the ancient name for haki. The reason Skypeia still call it mantra because they have been separated from the rest of the blue seas since ancient times.
If this idea is not in favor with you, just comment it politely. Constructive ideas are welcomed and share yours. Thanks for reading.