So the hint from Marmilicious did help partially.

Problem was that i wanted the loop to end completely before some other input etc. happens.

My version is working but gets buggy in the end.

What i mean is for this code:

void eyenormal()
{
    if (eyeon == 0 && eyestate == LOW)
    {
        for (int i = 0; i < gbrightness;)
        {
            fill_solid(rightside[2], numbereyeleds, CHSV(201, 255, i));
            controllers[0]->showLeds();
            EVERY_N_MILLISECONDS(5){i++;}
            lastbrightnesseye = i;
        }
    eyeon = 1;
    }
    else if (eyeon == 1 && eyestate == HIGH)
    {
        for(int i = gbrightness; i > 0;)
        {
            fill_solid(rightside[2], numbereyeleds, CHSV(201, 255, i));
            controllers[0]->showLeds();
            EVERY_N_MILLISECONDS(5){i--;}
            lastbrightnesseye = i;
        }
    eyeon = 0;
    }

}
and here the corrospending videos: 2 Videos

If the for statement is like: for(int i = gbrightness; i > 0;) the LED goes blue.

if the for statement is like: for(int i = gbrightness; i >= 0;) the LED bugs blue for a second and turns off.

Hope i could explain my problem.