fading leds on a guage

I want to put LEDs behind the instruments of my 97 dodge ram 1500, and I was wondering if there is a way to change the color of the of the LEDs with the guage, like on a tach, idling, the LEDs would be blue, and as the engine revved higher, all the LEDs would fade to red

Yep. So what's your preference - a uPC or PIC or Fusion Brain etc, or discrete (IC) electronics... The fading will be complicated for the electronic version (the color change isn't) unless perhaps multi-colored LEDs are used. I suggest however the first step it to mount the LEDs and find the right combinations and brightnesses etc. What you find there can have a big impact on the strategy. For digital solutions, it's easy - just add LED drivers and program accordingly. The LED drivers might be PWM (Pulse-Width Modulated) circuits to vary brightness and fading. There were also similar aftermarket gauges for cars and industrial applications (once upon a time?).