voltage and ampere

what is the difference when a bulb with specification 12v 1A, and i supply it with 12v 0.5A and 6v 1A?
i know the bulb will not bright, but is there any other difference? example, with 12 0.5A volt will produce larger spectrum then 6 volt 1A(guessing).
if their is no difference with a bulb, is there a difference with speaker, heater, or motor?

we can abstract the bulb as a resistor, so i think the 12v 1A means resistance of the bulb is 12 ohm.
1. 12V 0.5A you need to limit the current of the power supply, half power dissipation, maybe half light intensity ?
2. 6V 1A U=RI, so no this situation will happen.

I believe lawjet meant to say for "2." that 6A @ 1A will not happen since the bulb requires 12V to carry 1A. Current and voltage are not independent variables with a single resistive type load.

At 1A, the 12V/1A bulb will give wide-spectrum white light.
When its current is reduced to 0.5A by reducing the voltage fed to it then it will give narrow-spectrum dimmed reddish light.
At 6V it will be dim and red even more but its fairly low current will not drop as much as a resistor because when it is warm then its resistance is less than when it is hot.