For reference...

The forward voltage of an LED is an attribute of the semiconductor used to create it, as is the color of the LED. So most LEDs of the same color will have the same forward voltage, as they will mostly be using the same (cheapest to manufacture) semiconductor material.

This voltage is typically 1.7V for red, 2.1V for yellow and 2.2V for green. (All 'standard' LEDs not 'high efficiency' or 'super-bright'). These 'traditional' LEDs have forward voltages so close together that in the vast majority of applications where the output brightness is not required to be calibrated, engineers would simply use 2V as the forward voltage when calculating the series resistance. ( R=(Vsupply-Vforward)/ Iforward ). For a typical Iforward of 10mA this worked perfectly fine.

Newer developments have created LEDs with a wider range of Vforward. Blue and white LEDs are typically 4V or more, so that rule of thumb can't be used for all LEDs anymore.

There are no 12V LEDs. When you see something labelled as such, it is really a standard voltage LED with a built in series resistor calculated for a supply voltage of 12V. Such items are convenient for indicators in automotive and marine environments, which is exactly why they are made.

A PC motherboard has the series resistance built in. I'm not sure whether the ATX specs defined the colors used for the power and HDD LEDs or not. But I'd bet that most of the engineers used the 2V value anyway.