It is a simple enough question, and the answer can be simple enough too, but the simplicity of the question is independent of the answer.
The simple answer is, yes LEDs can vary intensity(brightness), depending on voltage/current.
The more complicated answer:
Hue is not realistically adjustable, but brightness(intensity) is.
Since this forum is about using LEDs with microcontrollers, using PWM is a viable way to control brightness, but it is not necessarily better for every situation.
LEDs have a usable range, where if you dont have enough electricity, it wont light up at all, and it gets to a point where it wont produce any more light, it just causes more damage. In a sense, they can be looked at as simple on or off, but they can also vary brightness to some degree.