Well lets tackle this question first
Is this a common thing to happens when it goes from being under one kelvin lamp to another?
Yes, this is so common that it's almost not funny
![Smile :) :)](https://cdn.jsdelivr.net/joypixels/assets/8.0/png/unicode/64/1f642.png)
Many aquatraders and the like keep things under 20000k bulbs, they aren't the best for growing conditions however they do make colors "pop" and corals will physically change colors, not to mention if this is a new shipment straight from the ocean they came from vastly different conditions (light, flow, nutrient levels) than the LFS's tank and your tank, so if you expect a coral to stay one particular color than you might be setting yourself up for disappointment
and will it regain its colour back?
Maybe. Being as you got this color a few days ago I'd say the reason for the color change happens to be the wavelength spread of your lights, in the same way some corals fluoresce under actinics if you get a coral from a tank like that and put it in a tank with no actinics there will be no fluoresce like it was. However blue really isn't a fluorescing color so this might just be stress of some sort, but 10kK bulbs do put out a bit more red/yellow than 20kK which is almost all blue/violets so what you see is simply what your lamps are doing. You can't put a red apple under a pure yellow lamp and expect it to still look red can you? So long story short, I think what you have here is what you should expect unless you switch to a bluer bulb.
That all being said, I don't think this is a "true" superman simply for the fact that stores are calling everything that is blue and has red polyps a "superman", although it does look like there's a couple green polyps mixed in, so you might tell everyone you have a green lantern & superman's love child coral!
![Big grin :D :D](https://cdn.jsdelivr.net/joypixels/assets/8.0/png/unicode/64/1f600.png)