To explain why during the other days of the year the Kohen Gadol served in gold and colored vestments and on Yom Kippur when he entered inside [the holy of holies] he served specifically in white vestments.
It is written (Tehillim 33:6) "By the word of Hashem heavens and earth were made," the Almighty created the worlds by His utterances and speech. All the worlds are filled with His life force and vitality and exist because of that life force and vitality. And if, heaven forbid, there would be a lack [of His life force] for even one moment it would be totally impossible for any of the worlds to exist so we find that all of the worlds exist through His life force.1
The shefa which the Creator sends down over all the worlds before it reaches the worlds it is without any shape or form or color. As the shefa spreads through the world then it becomes formed and colored. For example as the shefa spreads through the world of the fiery angels known as the seraphim then it takes on the form of the world of the seraphim and so it does with all the worlds wherever it spreads out it takes on that form of that world, so that for inanimate objects, plants and living animals then the shefa takes on the form and hue of the inanimate, the plants and the animals.
This is known as General and Specific. When the shefa has not yet spread out it is called general and when it has spread out over all the worlds, then it is called Specific. When the shefa has spread out over all the worlds then it has taken on the form and hue to match the world it is in and where it has spread out to, if it is shefa in the world of the seraphim then it takes on the form of that world of seraphim and it can no longer spread out past the world of seraphim. And this is true for all of the worlds. This is hinted at by the statement in (Eruvin 28a) "A General statement followed by a Specific one, the general can only contain what is in the specifics." This occurs when there is an awakening from above, however when there is an awakening from below to send forth shefa then the opposite is true. An awakening from below is always only Specific however the shefa which the Creator sends to the world is General and inclusive, including and incorporating everything and anything and this is what the aforementioned Gemarra (ibid) means, "A Specific statement followed by a General one, the general adds on to the specific."
This will explain that the Kohen Gadol on Yom Kippur draws down shefa to all of the worlds and this shefa when it is still being drawn down and has not yet entered the worlds, we explained that this shefa is completely colorless [and without form]. This hints at the reason that the Kohen Gadol specifically wears white vestments, which alludes to the drawing down of shefa which on Yom Kippur has no color at all. On the other days of the year his service was only to keep the shefa in the world as we explained that when the shefa in the worlds then that shefa has form and hue and that is why the rest of the year the Kohen Gadol wore gold and colored vestments.
See Tanya Shaar haYichud VaHaEmunah Chap 1
Story: Like The Avodah of the Kohen Gadol or an Angel?
The Tzanz Klausenberger Rebbe related:
"This story is what I was told by Rav Tevli of Dukla who heard it from his father-in-law Yosef Moshe, who heard from Rav Yitzchak Ettinger who heard from the mouth of the Gaon author of the author of the Yeshuos Yaakov himself. He related that when he (the Yeshuos Yaakov) was but a young avrech he lived in Yaroslav (which was a city of misnagdim or opponents of chassidus) and this is what he told:
"Once on Erev Yom Kippur the holy author of the Kedushas Levi the Rav of Berditchev arrived in town. Before Kol Nidrei he entered the shul crawling on his hands and feet in such awe and self negation before Hashem! He himself approached the amud, and from the lectern he began to recite Kol Nidrei with such awe and reverence. And at the time none of us assembled knew his identity and no one there had any idea who this guest was, nonetheless we let him continue leading the prayers before the congregation because he had the sweetest voice and we could tell by the tone and feeling that his prayers stirred that this was a person of great stature.
After the evening services concluded, he continued reciting the liturgy known as Shir HaYichud before the ark and afterwards he began the recitation of the entire sefer Tehillim out loud in voice that carried. He stayed this way on his feet the entire time all night long!
In the morning when the congregation began entering shul, we found him standing there just as before and he simply continued with Adon Olam, and led the prayers for Shacharis, he himself read from the Torah and continued to lead the services for Mussaf!"
"I," said the Yeshous Yaakov, "began to doubt whether this being was a human or angel from heaven! His voice and his unceasing never ending outpouring of strength and emotion were angelic. Yet when the congregation recited their responses and he stood silent I thought he must be human.
However when we reached the Neilah services and he began to roar out his prayers like a lion, the entire congregation was filled with remorse and all gathered did teshuva and repented. All of us present were sure that only a supernal angel from heaven could have such powers!
After the Maariv prayers concluded and Yom Kippur had ended, I decided to follow him and see were he was staying. I wanted to see if he would conclude his fast and eat as human beings do! Or was he an angel that needed not to eat nor sleep?
I watched as someone recited the havdalah service to which he listened to and immediately he began to ask those present, "please bring me something to enliven me and fill my hungry soul!"
Immediately they brought him some cake, cookies and fruit to eat, understanding that anyone would be hungry after such an intense day. "No, no," he explained declining the food offered, "this is not what I had in mind please bring a volume of meseches Sukkah from the Talmud."
They brought him the requested volume and he took the Gemarra and entered his room saying he needed to rest a bit. I followed him to see what rest meant and I peeked into his room and saw him sitting enraptured learning and studying Meseches Sukkah aflame in dveykus.
I myself could not hold up, I was tired and I fell asleep. When I awoke it was morning. Again I approached and saw how the holy Berditchever was still busy learning and close to finishing the entire tractate!"
'This,' interrupted the Tzanz Klausenberger, 'was the Kedushas Levi, about whom his son Rav Meir writes in the introduction to volume one of his sefer Keser Torah - "It is known to the entire world that my father had thousands of students whose hearts my father set aflame teaching them Gemarra, Rashi and Tosfos as well as the poskim and codes of law. Their hearts were excited to serve Hashem when they heard Torah from him guiding them on the straight path to serve Hashem."'
The Tzanz Klausenbeger concluded, "Even though he was busy serving and worshipping Hashem with such an avodah that would make even angels and fiery seraphs jealous, nonetheless he could not rest knowing that he did not study enough Torah on the day of YomKippur. Only after studying and completing the entire tractate of Meseches Sukkah was his mind appeased. What can we say about ourselves?"
(Shefa Chaim #22 parashas VaYelech p8-9 [p424-425])