Have a look at MarkT's posting above.
Now read the datasheets of some LED's to look for a chart of Voltage vs Current, etc.
The physics meaning of "red" is about 1.7 eV per photon, and surprise surprice a red LED starts to work when supplied with a bit over 1.7V. Likewise green is about 2.4 V and blue a little more.
Constant Current Driver
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One way to even up the outputs is to pick an "ultrabright" of each colour, and set up on pinboard to run each with "constant current" by placing a "bog standard npn bipolar transistor" such as a BC635 below it. 10kOhm from a d_out pin of the arduino at 5V supplies about 0.4mA to the gate of the npn. That defines the current I_ce through the npn to be about x40 that, which should be about right for the LED's; 16mA each irrespective of colour. You'd probably find that humans find the green most noticeable, so expect some tweaking from that by changing the 10k values a little. Lastly, to be kind to your npn, measure with a multimeter the actual voltage drop on each LED, which should be about 1/2 a volt more than its tun-on minimum. Choose resistors to go above each LED to 5V so that there is about 1V left to make sure that your npn is on.
For example 5V - (1V transistor +2.6V blue_minimum +0.5V blue_more ) = 0.9V surplus
16mA at 0.9V is about 55 Ohms above the blue. Inserting that should not change the behavior of the test circuit, but avoids much of the internal heating in the npn.
Lastly, and only do this with spare parts which you can afford to blow and well away from your arduino, try cooking an LED by supplying 10mA to the npn gate implying up to 400mA LED drive current and direct 5V with no extra resistor. You might get the LED so hot that its colour redshifts enough to notice. Look up 610nm orange and 614nm orange-red for an example of the small change to expect near to catastrophic overloading. It is worth doing that once, so that if you ever see it again you know the warning sign of "this LED is cooking and is about to blow". There is a certain small ammount of cooking which an LED will tolerate and go back to normal if left to cool down.