My very basic electronics education has taught me that parallel circuits are equivalent to separated circuits. To my surprise, when I was playing around with some electronics I found the following: 
Essentially I connected two red and two blue LEDs in parallel. The red ones lit up, but the blue ones didn't. Only when I removed the red LEDs would the blue ones light.
Why is this?
Answer
The red LED has a much lower voltage drop for a given current. In that way, the red ones light but the rest don't achieve their voltage necessary to light.
- Red LEDs have a voltage drop of about 1.8V.
- Blue LEDs have a voltage drop of about 3V.
You can see more colors and their corresponding voltage drop here in this table: http://en.wikipedia.org/wiki/Light-emitting_diode#Colors_and_materials
To solve this issue, you need a separate current limiting resistor for each led.
You could think about it as if you were putting two different zener diodes in parallel. If you have a 2 volt zener and a 5 volt zener, the 2 volt zener will reach it's voltage and prevent the 5 volt zener from ever passing any current.
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