The explanations that people are giving you are good but maybe a different one will help.
A LED will pull WAY more current than is healthy for it if it is not restricted in some way. The color is one indication that things are going badly and in short order. The resistor is sized so that it limits the current to a safe level using Ohm's law. Limiting it to MAX will reduce the potential life of the LED. Common practice it to limit it to less than max, hence, the suggestion of 30 mA instead of 50 mA. The voltage rating is what is needed to light the particular material (they are different because they use different materials to give different colors).
A quick look at a Digikey catalog gives a few examples:
Color,forward voltage, max current
Red, 2.0vdc, 25mA
Green , 2.2vdc, 25mA
Blue , 3.65vdc, 30mA
You can run any of these off of a 12 v supply because there is more than enough to overcome the "bandgap" of the material, but you still need to limit the current to around 20 mA. (less than the max listed).
The calculations are this:
Red: 12.0v - 2.0v = 10v: 10v / .020A = 500 ohm
Green: 12.0v - 2.2v = 9.8v: 9.8v / .020A = 490 ohm
Blue: 12.0v - 3.65v = 8.35v: 8.35 / .020A = 417.5 ohm
Common practice it to find the common value that is closest, but higher value, or if close, but lower (more current) recalculate and see how close you are to max and determine if you can live with the reduction in life.
On a side note, if you put a 1n917 diode accidentally across 120v ac mains, they release their smoke explosively! ie: glass shards many feet away.
Reading between the lines of your questions, prudent advise would be that lower voltage is much more likely to give you the chance to live a long and fruitful life.
I will admit that the idea did intrigue me when I first read it. Outdoor IR illumination came to mind, but after mulling it over, even with 25 years of working around the "magic lightning" I decided I'm going to pass.
Good luck
Bo
The little LED thumb sized flashlights are an interesting case in design. You open it and wonder why they don't need a resistor. The batteries are coin cells that are limited in the amount of current that they can supply even if shorted, so the current limit is a function of the batteries. If you supplied the same voltage from a power supply, the LED would smoke in short order.
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