Blue + Red = Purple
Colours, like numbers or sound, carry deep spiritual significance. Within the system of tarot, they bring many layers of meaning, much of which we won't even begin to scratch the surface of in this post. But, that's not to say we can't start peeling back those layers!
First, what is colour? Simply put, "colour is the visual perception based on the electromagnetic spectrum. Though color is not an inherent property of matter, color perception is related to an object's light absorption, reflection, emission spectra, and interference."*
I remember reading something in a book a few years back that really stuck with me as it relates to colour (though the name of the book didn't stick quite as well, unfortunately). It said that colour is the only thing in a drawing, painting or other visual interpretation that is not a symbol for something else - it is pure - it's simply always itself. For example, when we draw a picture of an apple, we know that's not actually an apple, but rather a signpost of sorts. But, when we colour that apple red, we can't say the same thing about the colour. The colour just is - it's not a symbol for something else. Because of this, colour is powerful in its ability to stir up emotions, which is why it is used very intentionally (and manipulatively) within marketing.
Two colours you will see a lot throughout tarot are red and blue. Red and blue are both primary colours, and symbolically they represent a certain dichotomy i.e. self vs sub consciousness, male vs female, sun vs moon, fire vs water etc.
The individual use of these two colours can communicate a lot about what a card is intending, but what happens when you mix them together? Well, you get purple (or violet) - literally. Violet is associated with the crown chakra, spirituality and wisdom...among other things. Another important thing to note about violet specifically, is that it is the colour with the highest vibrational speed. As such, it symbolizes union, divine marriage, the overcoming of opposites or said another way, a state of super (cosmic) consciousness.
While red and blue are used a lot in tarot (RWS), purple is rarely seen. And when we do see it, it's used in very intentional ways. A really interesting trio that plays with this blending is The Magician, The High Priestess and The World. (p.s. I've put The High Priestess first to make a point about the numerical connection, which we'll talk about below).
The High Priestess represents sub-consciousness, the lunar aspect, and wears a blue robe. The Magician represents self-consciousness, the solar aspect, and wears a red robe. The World on the other hand, is a precise blend of both of these forces, and as such, dons a purple cloth.
In The World, there is union, harmony and balance - a kind of yin/yang. It's also no coincidence that the world is the 21st key. The numbers of The High Priestess and The Magician make up the number 21. In chronological order, we get 12, but in reverse, we get 21. If you look at The Hanged Man with this in mind (key 12), you'll see the red and blue together (but separate), before they are fully harmonized in The World.
Side note: 1+2 also equals 3, which The World (2+1) deduces to.
With all the colours and numbers connecting these cards (1&2), it could be said that The Magician and The High Priestess come together in The Hanged Man, but fully meld into one another in The World.
Now, whether The World ultimately depicts a "pre fall" condition, an evolved "super conscious" state or something in-between, we can't say for sure. But, what we can say is that it symbolizes something that both of those possibilities entail: the dissolution of duality, perfection and oneness - all of which is made possible by the perfect blending of alchemical opposites, red and blue.
*https://en.wikipedia.org/wiki/Color