YES, YOU CAN USE 12 VOLT LEDs IN A 6 VOLT SYSTEM!
That is not universally true for 12V LED bulbs. While it it true that LEDs function primarily by current, they also have a minimum threshold voltage below which they will not flow any current or light up at all.
Once you get above that voltage, the brightness is a function of the current (which is also a funxtion of the voltage drop). Inexpensive bulbs will often be just a series of LEDs with an inline resistor. The current becomes your vehicle voltage (13.5VDC for a 12V system) minus the sum of the threshold voltages for the leds (variable, though often around 10VDC) divided by the resistance of the inline resistor.
If the sum of the threshold voltages is below your vehicle voltage, you will get some light out of the LEDs. If not, yiu won't. It sounds like the ones you bought had a fairly low threshold voltage so it would function with a 6V system. That is not always the case though.
There exist bulbs that have a proper current driver system in them, but they are much, much, much more expensive. With those, the voltage doesn't matter (as long as it is high enough for the converter) because the controller is converting it to a current supply. The ones I am the most familiar with operate from 4.5VDC to 28VDC and are the same brightness at all voltages.
