This is a question I had for some time in coming into comfort in working on my own soul. The indigo color, for me at least, was interestingly difficult to work with, as the rich blue-violet is darker in hue even in full brightness. If you have been frustrated with this, I promise you you are not alone in this.
Finding the color indigo online and meditating on that does help too, to get it logged as a color reference point in your mind.
I have spun these chakras and absorbed white gold and the similar indigo colored energy in to them, while also noting I have seen the third eye's color listed also as silver. When I spin the chakras at the speed of light, I see a color vibrating back and forth between a silver/gold white with a halo or outline of indigo around them. Like a bright sun is within the chakra(s) you are working on. Though I do try and maintain visualizing them as just indigo as well. Gradually the color indigo has been easier to visualize, and also the "black-light" energy of Aether, a little easier to see as well, this is most transparent and less solid I have experienced.
Since finding that the third eye chakra is also silver, I am putting in meditation time to make sure that it is meditated on in that way too.
I started with a periwinkle blue kind of color, as that is what it seemed like these 6th chakras would become as sped up in the spinning exercise, between white gold and indigo there may be a transitional area where this color can be found, but eventually the color gave way more to white gold with the halo/outline of indigo around the chakra when spinning, and the pure indigo color when at lower speeds or not spinning at all.