How to change an LED with a thermistor
The resistance of a thermistor changes with temperature.  In the datasheet for the thermistor there will be a graph that shows the resistance over some temperature range.  If you cannot get the datasheet for the termistor, don't buy that thermistor.  Digikey has datasheets for everything they sell, but eBay can be hit or miss.

Sometimes the resistance goes up as the temp increases.  This is called a Positive Temperature Coefficient, or PTC thermistor.  Other times the resistance decreases as the temp increases.  This is a Negative Temperature Coefficient, or NTC thermistor.

When a thermistor is advertised as being a 100K thermistor, that means that it has a 100K ohm resistance at some "normal" temperature-- usually room temperature.  The datasheet will tell you what the "normal" temp is.  Another reason why you should not buy one without the datasheet.

Next is the problem of figuring out how to control the RGB LED with the thermistor.  Judging from your question, I am assuming that you're new to electronics.  This is unfortunate, as the problem of controlling the LED can be difficult.  Especially if you want to control the color based on the temp.

One thing that you likely do NOT want to do is to use the thermistor as the current limiting resistor that is normally found with LEDs.  There are several reasons for this.  #1 is that the resistance of most thermistors is too high to be practical as a current limiting resistor.  Not impossible, just not usually practical.  #2 is that thermistors often cannot handle a lot of current flowing through them that is required for an LED.  Again, not impossible but you must carefully choose the thermistor.  #3 The change in resistance over the required temperature range is probably not right to get the desired look you seek.  And the big one is: #4  If you run a lot of current through the thermistor you will actually heat up the thermistor, thus corrupting your temperature measurement.  

There are several things you might want to control based on the temp.  The main ones are the intensity of the LED and the color of the RGB led.  Controlling the color is probably what you want, but it is the most difficult.  The reason why it is the most difficult is because the conversion from temp to R, G, and B is not linear (or even monotonic).  

The easiest way to do it is with a microcontroller.  Use the thermistor as one resistor in a voltage divider and feed that signal to the ADC input of an MCU.  Then some software in the MCU reads the temperature and figures out the proper duty cycle for PWM-ing the R, G, and B LED's.  I know, this isn't easy-- but it is the easiest way.  There are other methods to do this but they are much harder and/or do not provide the best visual appearance.