True, however, I didn't want any color limit.

I don't think the style is too simple - it might actually be too complex for the size, it's hard to make out many things in the image.

It looks like you used opacity effects to create the light and figures - this is a no-no on PixelJoint because it creates a massive amount of uncontrolled colours. You have a lot of colours in this image that are used for 1-2 pixels each as a result. Using opacity effects can be a good start, but it almost always needs cleaning up afterwards.