The colored switch on the receiver/dryer is the one which activates the electric fan(s) based on pressure. It should be either green or red - probably red. I don't have the specs for the red switch in front of me, but it switches the fans on at about 225PSI and off at 175PSI. I wouldn't expect to see high enough pressure this time of year to activate the fan, unless you live in Florida or some such.
A quick test is to unplug the leads which connect to the switch and short them together. The fan should switch on. (Don't forget to have the ignition switched on!) If the fan runs, you've got either a defective switch or low refrigerant charge.
If the fan does not run, then it's electrical troubleshooting time. To do this right, you'll need the maintenance CD and an inexpensive volt/ohm meter. Probably the two most common problems are a dead fan or a burned out resistor. The fan can be tested by directly connecting it to the battery. Testing the resistor (it's right next to the receiver/dryer) requires an ohm meter. There's also the possiblity of a bad relay, faulty ground, or even defective wiring.