The color of the third eye chakra is indigo because it is between the fifth and seventh chakras. You produce the color indigo by mixing blue and violet. The fourth chakra, the throat chakra (blue), and the seventh chakra, the crown chakra (violet), influence the state of the third eye chakra(sixth) hence why the color is indigo. The chakras are deeply connected; however, we will only be focusing on the sixth chakra, the third-eye chakra.
The third eye chakra is used to signify a few concepts and ideas that are interrelated. The third eye chakra is used to signify wisdom, the subconscious mind, the future, intuition, communication with the universe, perception, and at the very core, it is a link to the Brahman. Brahman is a Vedic Sanskrit word that is present in Hinduism. It is used to refer to the Ultimate Reality in the universe and the highest principle in the Universe. Brahman is at the center of it all. It is unchanging but is the facilitator of changes. Brahman is the infinite, pure, eternal truth. You can think of Brahman as the source and the third eye chakra as a link to the source. This is why the third eye chakra is linked to wisdom, intuition, and the future because being connected to Brahman is to be connected to the universe in its entirety.
The third eye chakra appears as a lotus flower with two white petals. The two petals represent psychic channels of connection which meet at the center before rising to the crown chakra, Sahasrara. Sahasrara is the seventh primary chakra, and it is located at the top or the "crown" of the head. It is deeply connected to the third eye chakra because it is what happens after the third eye chakra is open. Sahasrara denotes a pure consciousness. To get to the pure consciousness, you must be connected to Brahman. And to access the Brahman, you must do so through your third eye chakra. If this is confusing, you can think of the third eye chakra as a door to the Brahman and Sahasrara as becoming completely connected to Brahman.
In popular culture, the third eye chakra is seen as the mind's eye where consciousness is centered. Although this is true, the Sahasrara is the chakra that holds pure consciousness. It is through the Sahasrara that the other chakras begin to manifest. Having a clear and open third eye means having access to intuition, perception, consciousness, and wisdom. It is also the point at which we leave the duality of "I" and "Others" and realize that we are all connected. Nothing exists in isolation from the other. This is signified in Ajna's appearance. In some systems, the Ajna will also contain Ardhanarishvara, a half-male and half-female split in the middle. This deity represents the composition of Shiva and Parvati, two Hindu deities. They are said to represent the subject and the object. When the third eye chakra is activated, you transcend duality and are absorbed into the consciousness of the universe where everything is connected.
This is a question a lot of people ask, and it's important that it is addressed. The third eye chakra isn't "blocked"; it is simply either not accessible or out of alignment. All seven chakras should be aligned, and when they are not, this can cause a few issues. It is important to not think of the Ajna as a location that can be blocked; it is a cycle. When you're not properly aligned, the cycle still works, but it is working out of alignment with the other cycles.
There are few signs you can pay attention to. Some signs of an unaligned third eye chakra are dizziness, inability to focus, maladaptive daydreaming to escape reality, dissociations, mental fog, lack of confidence, overdependence on the physical world, and also the superego. If you're already spiritually inclined, you will notice a physical misalignment when partaking in spiritual practices.
You need to understand that you already have a third eye chakra; it's simply not aligned. To align your third eye chakra, you will want to have meditation practices. Sometimes worries and stress can cloud our minds and un-align our chakras. Simple meditation practices that emphasize focus and clearing the mind are great for the third eye chakra. When you sit in stillness, the issues will arise; instead of looking for a distraction like you would usually do, you must face them, observe them, and let them go through you. Repression of emotions is one of the fastest ways to cloud the third eye chakra.
Another method is movement. The practice of yoga is irrefutably tied to the seven chakras. There are certain yoga poses that encourage energy in the Ajna. Some of them are the dolphin pose, the fish pose, the child's pose, and a shoulder stand. You'll want to begin with normal sun salutations before moving into these poses. These poses will often entail having the location of the third eye chakra making contact with your mat or the floor. You may feel a rush of emotions when you're in these poses. Don't hide from them. Acknowledge them, feel them, and let them go.