This is a weird topic that came to my mind suddenly when after post new stuff in the site and took a look to the home page, I've noticed that many pics had the "Pink" color on it.

Is the Pink the best color for hypno theme or the most popular? Or there's another color that fits better than Pink in the pics?

I'd say pink/purple does it well for me, but I also enjoy seeing other colors from time to time. Really depends on the context too. Red or light blue for eyes can be nice, while an aura of some sorts around the person can be a multitude of colors, like even yellow or white.

What I personally somewhat dislike are deep black eyes as a sign of mind control, as well as whited out eyes. With both it feels less human (not even doll-like) for me.

For me it's contrast. Red-white spirals or black-gold spirals are great for this. But in my view nothing will ever be better than the classic black-white spiral.

In many pics I've seen that Light Blue eyes are used for Tech Control and Red eyes focused on Magic control. I think they fit in those context, but is so weird that Pink/Purple can fit in many contexts, maybe because it combines the Blue and Red colors.

A bright glowy green. I associate it with evil and mind control is inherently not a very nice thing to do so I like it when the two mix.

Interesting thought. Maybe we're just conditioned to accept certain colors as certain forms of control, with pink/purple being the universal "master" color, that always does the trick.

I'm no color theorist (though I do have a Tumblr) but I reckon that the specific pinks and purples used are unnatural so that may be part of their appeal since mind control is altering someone's natural state. More likely is that they are often seen as "feminine" colors (with pink also associate with love) and so fit in milieu of a femsub scene.

Even more likely is someone did a thing that people liked are are copycatting, just like how Kaa eyes are usually a specific combo.