Choose a good color combo for my alert message for a dark background.
This is what I have now:
I don't know... I just think that colors aren't good enough for the scenario. Of course, it depends of my context, but I think the reds are too heavy, painful and shocking for humans' eyes - don't you think the same?
I need some other harmonious color to indicate validation errors that fits well the background.
I think all you need to do is change your text color to White:
Any high contrast color would work based on your image. It does not have to be red or a form of red/pink. A yellow, orange, lighter blue, violet, etc would all work. The only thing I would do is avoid green to stay away from a "success" impression. You can easily taylor the alert to match any existing color scheme you may be using.
The key is to simply keep contrast high.
Obviously red works as well, but you need to retain the contrast between the red background and the text as well. Red text on a red background is simply not high enough in contrast.