Control RGB LED without microcontroller
This is the simplest circuit I can think of (2 parts):-







simulate this circuit &ndash; Schematic created using CircuitLab

Input = 5V Red
Input = 0  Green

The circuit works because the \$V_F\$ of a Red LED is generally much less than the \$V_F\$ of a Green or Blue LED. 

Here is a typical RGB LED datasheet: 

And here are the typical plots of \$V_F\$ vs. current:



When the transistor is "off", the Green LED will have a forward voltage of about 3.0V at 10mA. When the transistor is "on", the Red LED will have a forward voltage of less than 1.9V. Referring back to the Blue/Green graph, little current will flow through the Green LED at that forward voltage. 

This particular arrangement shows a common-cathode arrangement, however it could be flipped and a similar P-channel MOSFET used. In that case, 0V would select the Red LED, and +V in would select the Green LED.